Diagnostic imaging in pregnant and lactating patients

INTRODUCTION — Diagnostic imaging is sometimes necessary during pregnancy and lactation. The safety of diagnostic ultrasound during pregnancy and lactation is well established. However, other types of imaging evaluation may also be required. Although the safety of radiation exposure during pregnancy is a common concern, a missed or delayed diagnosis can pose a greater risk to patients and their pregnancies than any hazard associated with ionizing radiation. In many cases, the perception of risk to the fetus or infant is higher than the actual risk.

This topic will review issues related to the safety of diagnostic imaging other than ultrasound in pregnant and lactating patients. Other related issues are discussed separately:

●Safety of diagnostic ultrasound in pregnancy (see "Overview of ultrasound examination in obstetrics and gynecology", section on 'Safety')

●Overview of radiation-related risks of imaging (see "Radiation-related risks of imaging")

●Effects of radiation in pediatric populations (see "Clinical manifestations, evaluation, and diagnosis of acute radiation exposure", section on 'Pediatric considerations')

NOMENCLATURE — Any discussion of the effects of radiation requires knowledge of radiation nomenclature and dosimetry.

●Radiation-absorbed dose – The amount of energy from ionizing radiation deposited in any absorbing material (eg, tissue) is the radiation-absorbed dose or "rad." An absorbed dose of 1 rad means that 1 gram of material absorbed 100 ergs of energy from exposure to radiation.

The gray (Gy) is the radiation absorption dose measured in international (SI) units.

1 rad = 0.01 Gy = 10 mGy

●Equivalent dose – The equivalent dose reflects the biologic effect of radiation exposure on human tissue. The unit for measuring the equivalent dose is the roentgen-equivalent man (rem).

The sievert (Sv) is the radiation equivalent dose measured in SI units. Depending on the type of radiation (beta, gamma, alpha, or neutron), the absorbed dose may be the same as or lower than the equivalent dose.

1 rem = 0.01 Sv

●Effective dose – Some organs are more sensitive to radiation than others, and this difference is reflected by the effective dose. The effective dose is calculated by multiplying the equivalent dose to an organ by the tissue weighting factor for that organ.

The unit for measuring the equivalent dose to an organ is also the rem or Sv.

Additional details of radiation dosimetry are provided separately. (See "Clinical manifestations, evaluation, and diagnosis of acute radiation exposure", section on 'Measures of radiation'.)

BACKGROUND RADIATION AND OCCUPATIONAL EXPOSURE — In the United States, the average person is exposed to a radiation dose equivalent of approximately 3.1 mSv (310 mrem) whole-body exposure per year from natural sources [1].

The United States Nuclear Regulatory Commission recommends that occupational radiation exposure (ie, exposure related to employment) of pregnant patients not exceed 5 mSv (500 mrem) to the fetus during the entire pregnancy [2]. The dose equivalent to the fetus is the sum of the deep-dose equivalent to the pregnant patient and the dose equivalent to the embryo/fetus resulting from radionuclides in the embryo/fetus and radionuclides in the pregnant patient. External exposures are monitored using individual monitoring devices. When indicated, internal exposures are estimated by measuring the radiation emitted from the body or by measuring the radioactive materials contained in biological samples. (See "Clinical manifestations, evaluation, and diagnosis of acute radiation exposure", section on 'Biologic effects of radiation'.)

CHOICE OF IMAGING STUDY

How to choose — Ultimately, the choice of the optimal imaging study for a pregnant or lactating patient is based on the information needed in the specific clinical setting, but with consideration of the potential fetal risks from exposure to radiation or contrast agents, or the risks to the infant from exposure to isotopes or iodinated contrast agents transferred into breast milk. These risks are discussed below.

The choice of imaging study or studies is best made jointly by the clinical (medical, surgical, obstetric) providers and the radiologist, who can sometimes modify the technique to minimize fetal/infant risk without significantly compromising the information needed for maternal diagnostic evaluation and management.

The approach to common disease-specific imaging in pregnancy is discussed in this topic and in other related UpToDate content:

●Appendicitis (see "Acute appendicitis in pregnancy", section on 'Imaging')

●Breast cancer screening – The role of screening for breast cancer with mammography during pregnancy or lactation is discussed in detail below. (See 'Selected imaging studies' below.)

●Breast mass – Diagnostic imaging for a pregnant or lactating patient with a palpable breast mass usually begins with ultrasound; mammography is performed if sonographic findings are suspicious for malignancy. This is discussed in detail separately. (See "Gestational breast cancer: Epidemiology and diagnosis", section on 'Diagnosis and staging' and "Breast imaging for cancer screening: Mammography and ultrasonography", section on 'Abnormalities on mammography'.)

●Deep vein thrombosis (see "Deep vein thrombosis in pregnancy: Epidemiology, pathogenesis, and diagnosis")

●Dental radiography (see 'Selected imaging studies' below)

●Pulmonary embolism (see "Pulmonary embolism in pregnancy: Clinical presentation and diagnosis").

●Kidney stones (see "Kidney stones in adults: Kidney stones during pregnancy")

Timing

●Patients of reproductive age – All patients of childbearing potential should be asked if they could be pregnant at the time of radiologic examination [3]. If any doubt exists, the results of a pregnancy test should be obtained before proceeding.

The ideal time to schedule nonurgent imaging studies in patients of reproductive age is during the first 10 days of the menstrual cycle, as this will aid in avoiding imaging during an unrecognized pregnancy. However, delaying imaging to coincide with menses may result in a missed or delayed diagnosis and can pose a risk to the patient. Thus, we do not routinely wait to schedule imaging to coincide with menses.

Preconceptional ovarian exposure to diagnostic levels of ionizing radiation has no measurable effect on future fertility or pregnancy outcomes.

●Pregnant patients – In pregnant patients, imaging studies should be performed without regard to gestational age when the information is expected to provide information that is needed for care. The potential risks of the relevant radiologic imaging techniques and the potential risks of avoiding imaging (eg, nondiagnosis or inaccurate diagnosis, worsening disease) need to be considered on a case-by-case basis.

Multiple national and international organizations have written guidelines on imaging the pregnant patient. A comprehensive resource including 17 organizations and their 33 reports was published in 2011 [4]. In 2017, the American College of Obstetricians and Gynecologists (ACOG) published a committee opinion regarding "Guidelines for diagnostic imaging during pregnancy and lactation" [5]. The information in this topic is derived, in part, from these reports and can help in decision making.

●Lactating patients – As with pregnant patients, imaging studies are performed in lactating patients when medically indicated.

Ionizing radiation and iodinated contrast agents do not affect the breastfed infant nor the timing of the study. (See 'Radiography and computed tomography' below.)

By contrast, in nuclear medicine studies, lactation should be suspended for the period of time that radioactivity is present in milk, which depends upon the half-life of the specific agent. (See 'Lactation risks' below.)

Techniques for minimizing fetal exposure — Preimaging consultation with the radiologist can help to ensure that the information necessary for maternal diagnosis and management is obtained at the lowest possible fetal radiation dose. Various techniques can be used to help safeguard the fetus.

●Abdominopelvic plain radiography – The following techniques can be used to minimize fetal radiation exposure during studies in which the fetus is directly in the field of view:

•A posterior-anterior exposure lowers the fetal radiation dose by 0.02 to 0.04 mGy (0.00002 to 0.00004 Gy, 2 to 4 mrad) compared with the traditional anterior-posterior exposure because the uterus is located in an anterior pelvic position.

•Shutters can be employed to collimate the radiation beam and reduce scatter.

•Avoiding magnification near the uterus and use of grids decrease the fetal dose of radiation.

•Minimize repeat examinations.

●Computed tomography – The fetal radiation dose from a computed tomography (CT) scan is affected by several variables, including the number, location, and thickness of slices. When CT imaging is performed in pregnancy, using a narrow collimation and wide pitch (ie, the patient moves through the scanner at a faster rate) results in a slightly reduced image quality but provides a large reduction in radiation exposure. Scanning protocols should also be modified. As an example, if performing a CT scan with contrast, the number of acquisitions can be reduced by eliminating the precontrast series. (See "Principles of computed tomography of the chest".)

●Fluoroscopy and angiography – During fluoroscopic and angiographic imaging studies, modifying the exposure time, number of images obtained, beam size, and imaging area can reduce the amount of radiation exposure.

●Nuclear medicine – Maternal hydration and frequent voiding reduces fetal exposure to radionuclides excreted in the urine and accumulating in the maternal bladder.

●No role of shielding in nonabdominopelvic plain radiography – Shielding (eg, wearing a lead apron) is no longer routinely used during radiological imaging of pregnant patients; this is consistent with recommendations from several organizations including the American College of Radiology (ACR), American Association of Physicists in Medicine, and Health Physics Society [6-8].

Diagnostic radiographs of the head, neck, chest, and limbs (which do not include the fetus in the imaging field) produce almost no scatter to the fetus; thus, any radiation received does not result in a measurably increased risk of any adverse outcome. Furthermore, use of these shields may compromise the diagnostic efficacy of the exam and, as a result of radiation being reflected back towards the patient, actually may result in a small increase in radiation dose.

RADIOGRAPHY AND COMPUTED TOMOGRAPHY — Ionizing radiation includes radiography (eg, chest radiography, mammography, barium enema, cystourethrogram, fluoroscopy) and CT.

Maternal risks — Maternal radiation-related risks are generally similar to those in nonpregnant patients. However, pregnant patients experience proliferative changes in their breast tissue during pregnancy. Since proliferating breast tissue is more sensitive to radiation, it has been hypothesized that exposing the chest to ionizing radiation during pregnancy or lactation may lead to a small increase in the lifetime risk of breast cancer, but this has not been proven [9]. These issues are reviewed in detail separately. (See "Radiation-related risks of imaging" and "Clinical manifestations, evaluation, and diagnosis of acute radiation exposure".)

Fetal risks — There are no high-quality studies in pregnant humans from which to derive data on risks of ionizing radiation on the fetus; most of our information is based on case reports and extrapolation of data from investigations of survivors of the atomic bomb in Japan and the Chernobyl accident [10-16].

Types of effects — The health effects of ionizing radiation are typically categorized into two main categories:

●Deterministic effects – Deterministic effects only happen above a threshold dose and the severity of the effect increases as the dose increases above the threshold. A large number of affected cells results in more significant clinical problems. If injury to these cells occurs during a critical stage of organogenesis (primarily but not exclusively two to eight weeks after conception (figure 1)), impairment, agenesis, or deformity of the developing organ can occur. For example, microcephaly develops if a large number of differentiating central nervous system cells are injured. Pregnancy loss and growth restriction are other deterministic effects (table 1). (See 'Potential consequences' below.)

●Stochastic effects – Stochastic effects can occur at any radiation dose and result in changes to the cell genome and altered differentiation and function of the affected cells. The probability, but not the severity, of the effect increases with increasing radiation dose. Childhood cancer is the primary stochastic effect of fetal radiation exposure. As an example, the increased risk of thyroid cancer as a result of in utero exposure to radiation after the Chernobyl accident is a stochastic effect [10]. (See 'Potential consequences' below.)

Dose threshold — The threshold at which an increased risk of deterministic effects are observed in radiation-exposed embryos/fetuses has not been definitively determined; we use a threshold of 50 mGy as a conservative estimate.

●Dose <50 mGy –Diagnostic imaging studies typically expose the fetus to less than 50 mGy (0.05 Gy, 5 rads), and there is no evidence of an increased risk of fetal anomalies, intellectual disability, growth restriction, or pregnancy loss from ionizing radiation at this dose level (table 2) [17,18]. There may be a small increased risk of childhood cancer; this is discussed in detail below. (See 'Potential consequences' below.)

The margin of safety at levels below this threshold is augmented by the fact that most human exposures from diagnostic imaging will be fractionated over a period of time; this type of exposure is less harmful than a single large acute exposure [17].

●Dose >50 mGy – Diagnostic imaging studies, even those associated with higher fetal exposure (eg, abdominal or pelvic CT, barium enema, cystourethrogram), almost never expose the fetus to levels of radiation >50 mGy. Potential consequences of ionizing radiation to the fetus at such levels are discussed below. (See 'Potential consequences' below.)

Whether there is a risk between 50 and 100 mGy (0.05 to 0.1 Gy, 5 to 10 rads) is unclear [19]. However, evidence suggests the risk begins to increase at doses above 100 mGy (0.1 Gy, 10 rads), and particularly above 150 to 200 mGy (0.15 to 0.2 Gy, 15 to 20 rads).

In one retrospective study including 97 pregnant patients who underwent abdominal or lumbar radio-diagnostic imaging during the first trimester and in whom follow-up data were available, of the five patients exposed to 50 to 90 mGy, one congenital anomaly was observed; no congenital anomalies were observed in patients receiving <50 mGy, but numbers were too small to determine statistical significance [20].

Potential consequences — The potential deleterious consequences of ionizing radiation at doses >50 mGy can be divided into four main categories [21,22]: pregnancy loss, congenital malformation, disturbances of fetal growth or development, and mutagenic and carcinogenic effects. The occurrence of each outcome depends on the gestational age at the time of radiation exposure, the dose of radiation absorbed by the fetus, and the performance of fetal cellular repair mechanisms. In the absence of any of these findings, the presence of other types of malformations in humans should not be attributed to radiation exposure [17].

●Pregnancy loss – During the first 14 days after conception, the developing human is most sensitive to the lethal effects of ionizing radiation. During this period, the radiation-exposed pregnancy either survives undamaged or is resorbed (termed the "all or none" phenomenon) [23]. Radiation-induced teratogenesis, growth restriction, or carcinogenesis are not observed during this stage of development [17], presumably because of the pluripotent nature of each cell of the very early embryo. For human exposure, a conservative estimate of the threshold for death at this stage is more than 100 mGy rads (0.1 Gy, 10 rads) [24]. An embryonic dose of 1000 mGy (1 Gy, 100 rads) will likely kill 50 percent of embryos.

During the period of organogenesis (approximately 2 to 8 weeks after conception or 4 to 10 weeks after the last menstrual period), the embryo may be damaged as a result of radiation-induced cell death, disturbances in cell migration and proliferation, or mitotic delay [25], but lethality is rare.

●Malformation – During the period of organogenesis, radiation damage may result in congenital malformations, particularly of the central nervous system (eg, microcephaly, gross eye abnormalities). Microcephaly is the most frequently cited manifestation of radiation injury in utero [26]. For the developing fetus under 16 weeks of gestation, the threshold for possible congenital malformations as a result of prenatal radiation effects is approximately 100 to 200 mGy (0.1 to 0.2 Gy, 10 to 20 rads) [24]. After 16 weeks of gestation, the consensus of most researchers is that this threshold is much higher, at least 500 to 700 mGy (0.5 to 0.7 Gy, 50 to 70 rads). After approximately 20 to 25 weeks of gestation, the fetus is relatively resistant to teratogenic effects of ionizing radiation [27].

●Disturbances of growth or development

•Fetal growth restriction – Atomic bomb survivor data showed a permanent restriction of physical growth with increasing radiation dose, particularly above 1000 mGy [24]. This was most pronounced when the exposure occurred in the first trimester. A 3 to 4 percent reduction in height at age 18 occurred when the dose was greater than 1000 mGy (1 Gy, 100 rads).

•Developmental delay – Studies in survivors of the atomic bombing of Hiroshima demonstrated that the risk of intellectual disability was highest for radiation exposures at 8 to 15 weeks after conception [11]. The abnormalities were attributed to alterations in neuronal development. No cases of severe intellectual disability were identified in the children of atomic bomb survivors who were exposed prior to 8 weeks or after 25 weeks following conception. The risk appeared to be a linear function of dose, with a threshold of 120 mGy (0.12 Gy, 12 rads) at 8 to 15 weeks, and 210 mGy (0.21 Gy, 21 rads) at 16 to 25 weeks [12-15].

At 8 to 15 weeks, the average intelligence quotient (IQ) loss was approximately 25 to 31 points per 1000 mGy (per 1 Gy or 100 rads) above 100 mGy (0.1 Gy, 10 rads), and the risk for severe intellectual disability was approximately 40 percent per 1000 mGy above 100 mGy. By comparison, at 16 to 25 weeks, the average IQ loss was approximately 13 to 21 points per 1000 mGy at doses above 700 mGy (0.7 Gy, 70 rads), and the risk of severe intellectual disability was approximately 9 percent per 1000 mGy above 700 mGy.

●Mutagenic and carcinogenic effects

•Genetic mutations – Radiation may increase the frequency of naturally occurring genetic mutations; it does not induce mutations unique to this source. Small increases in the rate of genetic mutation are difficult to detect because the background rate of spontaneous mutation is already high (approximately 10 percent), recessive mutations take several generations to become apparent, and autosomal dominant mutations are rare [13].

There is no way to distinguish radiation-induced genetic mutations from similar conditions arising from other environmental exposures. Studies attempting to estimate the incidence of radiation mutagenesis have been based largely upon animal and plant experiments. Very few human data are available, apart from observations in the offspring of atomic bomb survivors. An increased risk of genetic disorders induced by ionizing radiation has not been demonstrated in any human population at any radiation dose [2,13,28]. However, a perturbation of the normal sex ratio of live births, with an increase in the male-to-female ratio, has been reported and thought to be secondary to dysfunction of the paternal X chromosome [29].

•Carcinogenesis – Animal data suggest that carcinogenic effects are most pronounced during late fetal development [18]. Low levels (eg, 10 to 20 mGy [0.01 to 0.02 Gy; 1 to 2 rad]) of in utero radiation exposure may increase the risk of childhood cancer, particularly leukemia, by a factor of 1.5 to 2 over the baseline incidence of approximately 1 in 3000 [5,17,30]. Similarly, newborn radiation exposure of 10 mGy (0.01 Gy, 1 rad) increases the lifetime risk of developing a childhood malignancy, particularly leukemia, from the background rate of approximately 0.2 to 0.3 percent to approximately 0.3 to 0.7 percent [31]. However, the carcinogenic potential of low-level radiation is controversial since nonirradiated siblings of these children also have a higher incidence of leukemia. Furthermore, children exposed in utero at the time of the bombings of Hiroshima and Nagasaki have not developed a significantly increased rate of cancer [32].

Solid cancer incidence rates have been examined among survivors of the atomic bombings of Hiroshima and Nagasaki who were in utero or younger than 6 years at the time of the bombings [33]. Both the in utero (2452 individuals) and early childhood (15,388 individuals) groups exhibited dose-related increases in incidence rates of solid cancers, but the lifetime risks following in utero exposure were much lower than for early childhood exposure. At age 50, those exposed in utero or as young children had an estimated excess absolute rate of 6.8 and 56 per 10,000 person-years per 1000 mGy, respectively. There was no increase in oncogenic risk for exposures less than 200 mGy (0.2 Gy, 20 rads).

Selected imaging studies

Examples of common studies — Examples of the estimated fetal exposures for common imaging studies involving ionizing radiation are listed in the table (table 2) [25,34,35]. It is important to recognize that radiography and CT imaging of regions far from the uterus minimally impact the fetus.

Although several fetal radiation dose tables are available, dosimetry calculations vary widely, which can be confusing for clinicians and patients. Therefore, when counseling a pregnant patient about the radiation risks associated with a diagnostic imaging study, the estimated dose for the specific fetus should generally be calculated by a radiologist or radiology physicist familiar with dosimetry. Factors to be considered include the number and type of projections, exposure time, distance between the target and the fetus, radiograph output, shielding (not recommended), and use of digital acquisition systems designed to limit dose.

Screening mammography — While age-appropriate breast cancer screening can typically be deferred until after pregnancy and lactation have been completed, the American College of Radiology (ACR) deems screening digital mammography and screening digital breast tomosynthesis usually appropriate during pregnancy and lactation [36].

The hormonal changes in pregnant and lactating patients may cause proliferation of ducts and lobules that result in increased density and nodularity of the breast parenchyma on mammography. These changes make it difficult to identify small nodules, asymmetries, and architectural distortion and thereby decrease the sensitivity of the examination, although not all patients undergo these changes in breast density.

Diagnostic imaging, rather than screening, is performed for a pregnant or lactating patient with a palpable breast mass or abnormal finding on screening mammography. This is discussed in detail separately. (See 'How to choose' above and "Gestational breast cancer: Epidemiology and diagnosis", section on 'Diagnosis and staging' and "Breast imaging for cancer screening: Mammography and ultrasonography", section on 'Abnormalities on mammography'.)

Dental radiography — Dental radiography may be used during both pregnancy and lactation. The radiation dose to the fetus from maternal dental radiography is minute, 0.0001 mGy (0.01 mrads) for an average study and is not considered harmful.

Although one population based case-control study found an association between antepartum dental radiography of >0.4 mGy (40 mrads) to the maternal thyroid and low birth weight (less than 2500 g) [37], this association is not consistent with findings from multiple other studies and is not biologically plausible [22]. Further investigation is needed before any change is made to the recommendations for dental imaging in pregnant patients.

Use of iodinated contrast materials

●Pregnancy – Iodinated contrast materials may be used in pregnancy when indicated. They do not appear to be teratogenic or carcinogenic [38].

However, iodinated contrast materials cross the placenta and can produce transient depressive effects on the developing fetal thyroid gland. Although the fetal thyroid begins to trap iodine in the first trimester and produces T4 and T3 by midgestation, clinical sequelae from brief exposures to iodinated contrast material in the second and third trimester have not been reported [39,40].

●Lactation – Lactation can be continued without interruption after the use of iodinated contrast materials. This is consistent with expert guidelines, including the ACR and American College of Obstetricians and Gynecologists (ACOG) [5,41].

The ACR estimates that <0.01 percent of the maternal dose of iodinated contrast is absorbed by the breastfeeding infant [41]. Most iodinated intravenous contrast agents are highly protein bound; as such, they are rapidly cleared from the maternal circulation (half-life <60 minutes) and are present only at very low levels in breast milk [42-44]. Moreover, contrast agents have low oral bioavailability, so the infant absorbs a minimal amount of iodine.

In a study of 10 newborns who received intravenous contrast media for urographic studies, thyroid function tests were checked at the time of the study and 10 and 30 days later [45]. No abnormalities were found, suggesting that even therapeutic doses of contrast media administered directly to infants do not affect infant thyroid function.

Historically, patients were counseled about the theoretical adverse effects of iodinated contrast materials and to express and discard milk for 24 hours after the imaging study. This also remains the position of most manufacturers in their package inserts. Thus, patients who remain concerned regarding any theoretical risk after counseling may express and discard milk for a set period, but this is not our practice.

NUCLEAR MEDICINE — Nuclear medicine studies include pulmonary ventilation-perfusion, thyroid, bone, and renal scans; they use a radioisotope bound to a chemical agent.

Fetal risks — The effect of radioisotopes on the fetus depends on maternal uptake and excretion, placental permeability, fetal distribution and tissue affinity, as well as the half-life, dose, and type of radiation emitted. Fetal exposure also results from proximity to radionuclides excreted into the maternal bladder; maternal hydration and frequent voiding can reduce this type of exposure. (See 'Techniques for minimizing fetal exposure' above.)

Substances that can localize in specific fetal organs and tissues, and thus may be of concern, include iodine-131 (I¹³¹) or iodine-123 (I¹²³) in the thyroid, iron-59 in the liver, gallium-67 in the spleen, and strontium-90 and yttrium-90 in the skeleton.

Selected imaging studies — Examples of the estimated fetal exposures for some common imaging studies involving nuclear medicine are listed in the table (table 2). Selected studies that may be used in pregnancy are briefly discussed here.

Thyroid scan — By the 10^th to 12^th week of gestation, radioiodine isotopes are readily absorbed by the fetal thyroid. Although there are no reports of adverse fetal effects from diagnostic doses of radioactive iodine, it should not be administered to pregnant patients because induction of thyroid cancer in the offspring is a concern [25]. If a diagnostic scan of the thyroid is required, the preferred agent is Technetium-99m or I¹²³ (avoid I¹³¹) [5].

Positron emission tomography — There is minimal information regarding positron emission tomography (PET) in pregnancy. This technique involves injection of a radioisotope, fluorodeoxyglucose F 18. Animal reproduction studies have not been conducted with fluorodeoxyglucose F 18 Injection, and it is not known whether fluorodeoxyglucose F 18 Injection can cause fetal harm when administered to a pregnant patient or can affect reproduction capacity.

Because of the lack of safety data in human pregnancy, MRI or CT are generally preferred to PET as they usually provide similar information, but the decision needs to be made on a patient-specific basis.

Contact with others receiving radioisotopes — Pregnant patients may have contact with individuals who have received radioactive materials as part of a diagnostic study; the minimal residual radioactivity does not result in a measurably increased risk to the fetus. Radiation exposure from close contact is higher after some types of therapeutic radiation (eg, radioiodine therapy of thyroid cancer, brachytherapy implants for prostate cancer) [46,47]. A period of restricted contact may be prudent, depending on the type of therapy and dose administered.

Lactation risks — In nuclear medicine studies, lactation is often suspended for the period that radioactivity is present in milk; this depends upon the specific nucleotide used and the half-life of the agent [48]. The agent with the shortest half-life should be used. As many agents are renally excreted, increased hydration and frequent bladder emptying may decrease maternal exposure.

Before receiving the agent, the patient is typically instructed to express milk and store it in a freezer for later use. After the study, the patient should continue to express milk to maintain production, prevent engorgement, and store the milk until radioactive decay can occur. Radiology departments can screen milk samples for residual radioactivity before the patient resumes breastfeeding [49].

For patients who receive I¹³¹, I¹²⁵, N²², and Ga⁶⁷, the Committee on Drugs of the American Academy of Pediatrics advises interruption of lactation for a minimum of three weeks [50]. Because the lactating breast has a greater I¹³¹ affinity than the nonlactating breast and radioactivity persists after imaging, patients should stop lactation at least four weeks before whole-body scans with I¹³¹ and should not resume lactation thereafter to reduce the radiation dose, and potential cancer risk, to maternal breast tissue. A period of restricted contact with the infant may also be prudent, depending on the type of agent and dose administered.

Precautions for patients receiving I¹³¹ are discussed in more detail separately. (See "Differentiated thyroid cancer: Radioiodine treatment", section on 'Posttreatment precautions'.)

MAGNETIC RESONANCE IMAGING — Magnetic resonance imaging (MRI) uses electromagnetic radio waves, rather than ionizing radiation, to generate detailed images.

Safety — MRI is safe during pregnancy and, in some cases, is the preferred diagnostic modality because it provides better images than ultrasonography while avoiding the ionizing radiation of CT. As an example, first-trimester MRI is a reasonable option in a pregnant patient with suspected appendicitis in whom the appendix cannot be visualized by ultrasound examination. It is also sometimes used during pregnancy specifically to image the fetus or placenta. One expert panel on MRI safety stated that MRI should be performed at any stage of pregnancy when the information requested from the MRI study cannot be acquired by other nonionizing studies, the care of the patient or fetus during the pregnancy would be potentially affected by the data, and does not need to be delayed until the patient is no longer pregnant [51].

There have been no reports of adverse maternal or fetal effects from MRI during pregnancy [52-55], despite the following theoretical concerns: induction of local electric fields and currents from static and time-varying magnetic fields; radiofrequency radiation resulting in heating of tissue; trauma from projection of metal objects into the magnetic field; interference with the operation of electronic devices (eg, cardiac pacemakers) or position of metallic implants; burns from heating of conductive materials in implants; acoustic damage from high-intensity noise.

We believe it is probably safe to image at 3 Tesla for neurologic imaging of the maternal head/neck, but only if there will be a clear advantage in diagnostic quality (ie, at the recommendation of a neuroradiologist). However, almost all safety studies have been performed predominantly at or below 1.5 Tesla magnetic field strengths. In a retrospective study including over 1.4 million maternal-child pairs, those whose mothers underwent MRI in the first-trimester (1737 infants) compared with no MRI had similar rates of stillbirth, neonatal death, congenital anomaly, neoplasm, or vision or hearing loss in children followed up to age four years when adjustments were made for differences between exposure groups [55]. Magnetic field strengths were not reported but were likely at or below 1.5 Tesla since most cases can be done adequately at this level. Similarly, in a subsequent retrospective study including 81 neonates exposed in utero to 3 Tesla during MRI for maternal or fetal indications and matched 1:2 to unexposed controls, mean birth weight and prevalence of hearing impairment by 12 months of age were similar between groups [56]. There may be an increased risk of tissue heating at higher field strengths. For example, in an animal study at 3 Tesla, heating effects were shown in amniotic fluid and fetal tissue [57].

Use of gadolinium — Gadolinium is the contrast agent most used for MRI. It increases the signal from tissues that have increased blood flow, particularly in the setting of inflammation or neoplasm. In the mother, it is used for MRI evaluation of the brain and spinal cord, suspected inflammatory joint disease, inflammatory bowel disease, and inflammatory and neoplastic conditions of solid organs. It may also be useful for evaluation of inflammatory and neoplastic conditions of bone, muscle, and connective tissue.

●Pregnancy – Gadolinium should generally be avoided in the pregnant patient unless its use significantly improves diagnostic performance and is likely to improve patient outcome [5,58,59]. Gadolinium crosses the placenta and is excreted by the fetus into the amniotic fluid. It is then swallowed; thus, it can be reabsorbed into the fetal circulation.

Very low levels of gadolinium have been reported in fetal tissues (predominately bone) after in-utero exposure [59]. Human data from gadolinium exposed pregnancies are limited but concerning [55,60-64]. In the retrospective study including over 1.4 million maternal-child pairs discussed above (see 'Safety' above), those receiving gadolinium during MRI in the first-trimester (397 patients) compared with no MRI had an increased risk of rheumatologic, inflammatory, or infiltrative skin conditions (adjusted hazard ratio 1.36, 95% CI 1.09-1.69) and stillbirths and neonatal deaths (7 versus 9844 events; adjusted relative risk 3.70, 95% CI 1.55-8.85) [55]. Rates of nephrogenic systemic fibrosis (NFS, a rare disorder that usually occurs in adults and can be misdiagnosed as a connective tissue or skin disease in young children exposed to gadolinium in utero) and congenital anomalies were similar between groups. By contrast, in a subsequent retrospective study including almost 6000 pregnancies, those receiving MRI with and without gadolinium had similar rates of fetal and neonatal deaths (1.4 percent in both groups) [65].

The studies above involve a previous generation of gadolinium-based contrast agents [55,65]. The newer generation is more stable and may have a lower potential for fetal toxicity, but no data are available.

●Lactation – Lactation can be continued without interruption after the use of gadolinium; gadolinium-based contrast agents are present at very low levels in human milk and not absorbed well by the infant gut; no adverse effects have been reported in infants exposed through lactation [66-68].

In their statement on administration of contrast medium to lactating mothers, the American College of Radiology (ACR) estimates that less than 0.0004 percent of gadolinium-based contrast is absorbed by the breastfeeding infant. The ACR concluded that lactation is safe after receiving contrast media, but mothers should be informed of the theoretical risks of direct toxicity or allergic reaction [69,70]. The American College of Obstetricians and Gynecologists (ACOG) also concluded that lactation should not be interrupted after gadolinium administration [5]. Similarly, the Contrast Media Safety Committee of the European Society of Urogenital Radiology concluded that lactation may be continued normally when macrocyclic gadolinium-based contrast agents are administered [71].

Despite this guidance, patients who are concerned about theoretical adverse effects may pump to remove breast milk before administration of the contrast agent and then express and discard milk for 24 hours after the imaging study, which is also the position taken by most manufacturers in their package inserts.

Data evaluating the impact of prepregnancy exposure to gadolinium on pregnancy and lactation is limited [72].

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Ultrasound imaging in pregnancy" and "Society guideline links: Non-ultrasound imaging in pregnancy".)

SUMMARY AND RECOMMENDATIONS

●General principles – Diagnostic imaging is sometimes necessary during pregnancy and lactation. The safety of diagnostic ultrasound during pregnancy and lactation is well established. However, other types of imaging evaluation may also be required. (See 'Introduction' above.)

In many cases, the perception of risk of radiation exposure to the fetus or infant is higher than the actual risk, and concern about the possible effects of ionizing radiation should not prevent medically indicated diagnostic imaging studies using the best available modality for the clinical situation; a missed or delayed diagnosis can pose a greater risk to patients and their pregnancies than any hazard associated with ionizing radiation. (See 'Introduction' above.)

●Timing – All patients of childbearing potential should be asked if they could be pregnant at the time of a radiologic examination. If any doubt exists, a pregnancy test should be obtained prior to the diagnostic imaging study. (See 'Timing' above.)

In pregnant patients, imaging studies should be performed without regard to gestational age when the information is expected to provide information that is needed for care. Similarly, imaging studies are performed in lactating patients when medically indicated. (See 'Timing' above.)

●Minimizing fetal exposure – When imaging studies requiring ionizing radiation are necessary, various techniques can often be employed to minimize the radiation dose without significantly compromising the information needed for maternal diagnostic evaluation and management. (See 'Techniques for minimizing fetal exposure' above.)

Shielding (eg, wearing a lead apron) is no longer routinely used during radiological imaging of pregnant patients; this is consistent with recommendations from expert group organizations. (See 'Techniques for minimizing fetal exposure' above.)

●Radiography and computed tomography

•Dose threshold and potential consequences – Ionizing radiation studies typically expose the fetus to <50 mGy (0.05 Gy, 5 rads) (table 2), and there is no evidence of an increased risk of fetal anomalies, intellectual disability, growth restriction, or pregnancy loss from ionizing radiation at this dose level; there may be a small increased risk of childhood cancer. (See 'Dose threshold' above.)

Even studies associated with higher fetal exposure (eg, abdominal or pelvic CT, barium enema, cystourethrogram), almost never expose the fetus to levels of radiation >50 mGy. Potential consequences of ionizing radiation to the fetus at such levels include pregnancy loss, congenital malformation, disturbances of fetal growth or development, and mutagenic and carcinogenic effects. (See 'Potential consequences' above.)

•Use of iodinated contrast – Iodinated contrast materials may be used in pregnancy and lactation when indicated. They do not appear to be teratogenic or carcinogenic and lactation can be continued without interruption. (See 'Use of iodinated contrast materials' above.)

●Nuclear medicine – The effect of radioisotopes on the fetus depends on maternal uptake and excretion, placental permeability, fetal distribution and tissue affinity, as well as the half-life, dose, and type of radiation emitted. Examples of the estimated fetal exposures for some common imaging studies involving nuclear medicine are listed in the table (table 2). (See 'Fetal risks' above.)

For lactating patients undergoing nuclear medicine scans with radioisotopes, lactation is often suspended for the period of time that radioactivity is present in milk; this depends upon the specific nucleotide used and the half-life of the agent. (See 'Lactation risks' above.)

●MRI – MRI can be performed at any stage of pregnancy when the information requested from the study cannot be acquired by nonionizing imaging studies, and the data are needed to care for the patient or fetus during the pregnancy. (See 'Safety' above.)

•Use of gadolinium – Gadolinium should generally be avoided in the pregnant patient unless its use significantly improves diagnostic performance and is likely to improve patient outcome. Gadolinium crosses the placenta and is excreted by the fetus into the amniotic fluid. It is then swallowed; thus, it can be reabsorbed into the fetal circulation. (See 'Use of gadolinium' above.)

Lactation can be continued without interruption after the use of gadolinium; gadolinium-based contrast agents are present at very low levels in human milk and not absorbed well by the infant gut; no adverse effects have been reported in infants exposed through lactation. (See 'Use of gadolinium' above.)