Twin pregnancy: Routine prenatal care

INTRODUCTION — Many aspects of routine prenatal care and counseling of patients with twin pregnancies are the same as in singleton pregnancies (see "Prenatal care: Initial assessment" and "Prenatal care: Second and third trimesters" and "Prenatal care: Patient education, health promotion, and safety of commonly used drugs"). However, twin pregnancy is associated with higher rates of almost every potential complication of singleton pregnancy, with the exceptions of postterm pregnancy and macrosomia, and it is also associated with some unique complications. For this reason, some authorities have proposed specialized antenatal care clinics to address the specific needs of patients with twins; however, these clinics have not been proven to improve birth outcomes compared with standard care, albeit data from randomized trials are sparse [1].

This topic will review our approach to routine prenatal care of patients with twin pregnancies. This approach is generally consistent with recommendations of major medical organizations worldwide (see 'Society guideline links' below). Other important issues related to twin pregnancy are reviewed in detail separately:

●(See "Twin pregnancy: Overview".)

●(See "Twin pregnancy: Management of pregnancy complications".)

●(See "Twin pregnancy: Labor and delivery".)

●(See "Monoamniotic twin pregnancy (including conjoined twins)".)

●(See "Twin-twin transfusion syndrome: Screening, prevalence, pathophysiology, and diagnosis" and "Twin-twin transfusion syndrome: Management and outcome".)

●(See "Selective fetal growth restriction in monochorionic twin pregnancies".)

●(See "Twin anemia-polycythemia sequence (TAPS)".)

●(See "Multifetal gestation: Approach to delayed-interval delivery".)

DIET, SUPPLEMENTS, AND PHYSICAL ACTIVITY

Gestational weight gain — The National Academy of Medicine (formerly the Institute of Medicine) made the following provisional recommendation for cumulative gestational weight gain by term for patients carrying twins [2]:

●Body mass index (BMI) <18.5 kg/m² (underweight) – No recommendation due to insufficient data.

●BMI 18.5 to 24.9 kg/m² (normal weight) – 37 to 54 lb (16.8 to 24.5 kg).

●BMI 25.0 to 29.9 kg/m² (overweight) – 31 to 50 lb (14.1 to 22.7 kg).

●BMI ≥30.0 kg/m² (obese) – 25 to 42 lb (11.4 to 19.1 kg).

The National Academy of Medicine did not make separate recommendations for different classes of obesity, but another group used data from the US National Center for Health Statistics to suggest optimal cumulative weight gains of 22 to 44 lbs (10.0 to 19.9 kg) for class 1 obesity, 16 to 38 lbs (7.5 to 17.4 kg) for class 2 obesity, and 11 to 22 lbs ( 5.0 to 9.9 kg) for class 3 obesity [3]. The study population included over 260,000 pregnant people between 18 and 45 years of age who gave birth at 24 and 42 weeks of gestation to live-born twins without congenital anomalies from 2014 to 2018. Optimal cumulative weight gain was defined as the threshold below or above which the risk for an adverse perinatal outcome increased.

These thresholds represent the 25^th through 75^th percentile weight gains in patients who gave birth to twins weighing at least 2500 g [4]. Patients who met these guidelines generally had fewer preterm births and higher birth weights compared with those who did not meet the minimum weight gain suggested by the guidelines, while exceeding the guideline generally increased maternal risks of gestational hypertension/preeclampsia, eclampsia, and cesarean birth [5,6]. Poor gestational weight gain after 20 weeks appears to have a greater impact than poor first-trimester weight gain [7]; thus, we reassure patients in early pregnancy who are unable to achieve caloric intake or weight gain targets due to nausea and vomiting or other reasons.

To achieve appropriate gestational weight gain, a patient beginning pregnancy with a BMI in the underweight or normal range needs to increase their dietary intake by approximately 300 kcal/day above that for a singleton pregnancy or 600 kcal/day above that for a nonpregnant female. After 20 weeks of gestation, weight gain should be approximately 1.75 pounds (0.8 kg)/week for underweight pregnant persons and approximately 1.5 pounds (0.7 kg)/week for normal weight pregnant persons, with slightly lower weekly weight gain in pregnant persons who are overweight or obese.

Nutrition and supplements — We follow Society for Maternal-Fetal Medicine twin pregnancy caloric requirement and supplement recommendations, as shown in the table (table 1) [8].

Physical and sexual activity, exercise — In early pregnancy, patients with uncomplicated twin pregnancies can usually follow the same exercise/physical activity recommendations as those with singleton pregnancies as a definitive association between preterm birth and exercise or physical activity in twin pregnancies has not been established. We do assess for individual risk factors for preterm birth and counsel the patient accordingly.

As pregnancy progresses, physical changes generally limit the duration and type of exercise performed. Although case reports describe elite athletes who trained until giving birth despite a twin pregnancy, some restriction of activity is generally desired and prudent. Recommendations for individual patients depend on factors such as overall state of health and fitness, proposed exercise regimen, and musculoskeletal factors. (See "Exercise during pregnancy and the postpartum period".)Patients often inquire about work and sexual activity during pregnancy. Guidance is the same as that for patients with singleton pregnancies, with no restrictions for uncomplicated pregnancies. However, patients should be aware that working >28 hours/week and irregular working times (eg, working in the evening and/or at night) at 16 to 20 weeks of gestation increased the risk of preterm birth <32 weeks in a prospective observational study [9]. Little or no freedom in performing tasks and high physical workload were additional risk factors for very preterm birth in patients working >28 hours/week. For patients with complications such as vaginal bleeding, short or dilated cervix, placenta previa, preterm labor, or vasa previa, restrictions on activity are prudent and reasonable.

PREECLAMPSIA PREVENTION — Twin pregnancy is an indication for low-dose aspirin preeclampsia prophylaxis, since patients with twins are at increased risk for developing preeclampsia. The dose is the same as that in singleton pregnancies. Dosing and evidence of efficacy are reviewed separately. (See "Preeclampsia: Prevention", section on 'Low-dose aspirin'.)

SCREENING — Patients with twin pregnancies are offered the same screening tests as those with singleton pregnancies, with some differences, which are discussed below. (See "Prenatal care: Initial assessment" and "Prenatal care: Second and third trimesters".)

Screening for Down syndrome (trisomy 21)

Risk based on maternal age and zygosity

●For dizygotic twin pregnancies, the risk of at least one affected fetus is assumed to be twice the risk of a maternal age-matched singleton pregnancy (eg, at age 35: 2 x 1/350 at delivery) and the risk of two affected fetuses is assumed to be the product of the risk of the maternal age-matched singleton pregnancy (eg, at age 35: 1/350 x 1/350 at delivery).

●A monozygotic twin pregnancy is thought to have the same Down syndrome risk as a maternal age-matched singleton pregnancy (table 2); both fetuses will be affected or both fetuses will be unaffected. Rarely, however, the twins have different genotypes due to postzygotic events, including nondisjunction and mosaicism [10-15]. They can also be discordant for X-inactivation (in females), differential gene imprinting, and smaller scale genetic abnormalities, such as microdeletions [16].

However, at least one study reported that the observed incidence of Down syndrome in twin pregnancies is less than expected, possibly due to an increased frequency of early fetal loss [17]. In this study, the observed incidence of Down syndrome in dizygotic twin pregnancies was 75 percent of the expected incidence; in monozygotic twins, the observed incidence was 34 percent of expected. More data are needed before adjusting Down syndrome risk estimates.

Sesquizygotic twins are an extremely rare intermediate between monozygotic and dizygotic twinning. (See "Twin pregnancy: Overview", section on 'Relationship between chorionicity, amnionicity, and zygosity'.)

Test options — For patients who choose to undergo screening for Down syndrome, we suggest either the first-trimester combined test or cell-free DNA. While the combined test provides early, fetus-specific risk assessment, cell-free DNA has higher detection rates.

●Noninvasive screening using cell-free DNA – Noninvasive prenatal screening for Down syndrome using cell-free DNA is more complicated than in singletons because the average fetal fraction for each twin is less than that for singletons (although the total fetal fraction is higher in twin pregnancies) and the fetal fraction can differ between twins by at least 1.5-fold in 10 percent of cases [18]. Cell-free DNA findings may also be affected by early loss of one or more embryos of a multiple gestation.

Testing is commercially available for trisomies 21, 18, and 13. Although less validation data are available from twin gestations than from singletons [19-22], increasing evidence suggests that the sensitivity of the cell-free DNA test for Down syndrome in twins is similar to that in singleton pregnancies [23-26]. In a multicenter study that evaluated cell-free DNA results for trisomy 21 in 1447 twin pregnancies, trisomy 21 was detected in 41 of 42 pregnancies (97.6 percent sensitivity), with no false-positive cases; trisomy 18 was detected in all 10 affected pregnancies (100 percent sensitivity), with one false-positive case; and trisomy 13 was detected in four of five cases (80 percent sensitivity), with no false-positive cases [27].

Studies also generally report a higher rate of test failure in twins [23-25], but exceptions exist [26]. In the multicenter study described above, the nonreportable rate was 3.9 percent [27]. When a cell-free DNA test failure occurs, ultrasound and diagnostic testing should be offered but if time is sufficient, a second sample draw may also be considered [18].

The American College of Obstetricians and Gynecologists (ACOG) and The International Society for Prenatal Diagnosis (ISPD) provide the following guidance:

•A 2020 ACOG practice bulletin states that cell-free DNA screening for Down syndrome can be done in twin pregnancies, and available performance data are encouraging, but the total number of reported affected pregnancies is small, thus preventing an accurate assessment of performance [28].

•A 2021 ISPD position statement on cell-free DNA screening for Down syndrome in multiple gestations concluded that cell-free DNA screening for common trisomies in twins provides higher positive predictive values compared with serum- and nuchal translucency-based screening tests [18]. Test failures occur in a median 3.6 percent of cases (range 1.6 to 13.2 percent), which is higher than the rate in singletons and usually due to a low fetal fraction. (See "Prenatal screening for common aneuploidies using cell-free DNA", section on 'Twins'.)

Patients who screen-positive (ie, high risk) should be offered chorionic villus sampling (CVS) or amniocentesis for diagnostic genetic testing.

●First-trimester combined test – Increased nuchal translucency (NT) is a marker for Down syndrome, other aneuploidies, congenital malformations, and development of twin-twin transfusion syndrome (TTTS). The first-trimester combined test, which consists of ultrasound measurement of NT plus serum markers (beta human chorionic gonadotropin [beta-hCG] and pregnancy-associated plasma protein A [PAPP-A]) can provide fetus-specific risk assessment for Down syndrome in twin pregnancies. (See "First-trimester combined test and integrated tests for screening for Down syndrome and trisomy 18", section on 'Combined test'.)

Interpretation of maternal serum markers alone is problematic in twin pregnancies since both twins contribute to the concentration of the markers and marker levels may be affected by early loss of one or more embryos of a multiple gestation [29,30]. NT measurement can improve the detection rate and, importantly, help to identify which fetus is affected [31,32]. In a systematic review of first-trimester combined risk assessment (NT and maternal serum markers) in twin pregnancies, test sensitivity in dichorionic twins was 86 percent (95% CI 73-94) and test sensitivity in monochorionic twins was 87 percent (95% CI 53-98) [33]. At our institution, first-trimester combined risk assessment identified all six affected pregnancies (five discordant and one concordant for Down syndrome) at a screen-positive rate of 5 percent, while NT alone detected five of six affected fetuses [32]. Although serum marker testing enhanced risk assessment, the levels in affected twin pregnancies were closer to the median levels than in affected singleton pregnancies.

Of note, the false-positive rate of NT screening is higher in monochorionic than dichorionic twins because increased NT can be an early manifestation of twin-twin transfusion syndrome as well as a marker of aneuploidy [34]. Also, in vitro fertilization (IVF) affects serum marker levels used in Down syndrome screening and may be considered by some laboratories when calculating screening results in twins conceived by this method [35]. (See "Maternal serum marker screening for Down syndrome: Levels and laboratory issues", section on 'IVF and other infertility treatments'.)

Also of note, fetuses with the same genotype may have different phenotypes; as an example, only one fetus of twins with Down syndrome may have increased NT. In a series of eight monochorionic twin pairs discordant for NT who were karyotyped, discordance was a marker for concordant chromosome abnormalities in one twin pair and discordant chromosomal abnormalities in two twin pairs [15].

Patients who screen-positive (ie, high risk) should be offered amniocentesis for diagnostic genetic testing. A second screening with a cell-free DNA test is another option because of its higher sensitivity and lower false-positive rate, but the challenge of screening both fetuses from one maternal sample and the limitations of screening (<100 percent sensitivity and specificity) remain.

Diagnostic testing — Both fetuses should be karyotyped when karyotyping is performed since even monozygotic twins may be discordant. Issues related to diagnostic genetic testing in twins by CVS and genetic amniocentesis are reviewed separately. (See "Chorionic villus sampling", section on 'Multiple gestations' and "Diagnostic amniocentesis", section on 'Multiple gestation'.)

Screening for congenital anomalies — An anatomic survey is typically performed at 18 to 20 (or 22) weeks of gestation in all twin pregnancies [36], as well as all singleton pregnancies [37]. The incidence of congenital anomalies is three- to fivefold higher in monozygotic twins than in dizygotic twins or singletons and is higher in monozygotic monochorionic twins than in monozygotic dichorionic twins [38-42]. In one study, the frequency of congenital anomalies in monochorionic twins was 634/10,000 versus 344/10,000 for dichorionic twins versus 238/10,000 for singletons [41].

The concordance rates for any congenital malformation in monozygotic and dizygotic twin pairs was 37 and 17 percent, respectively, in a study of nearly 3400 twin pairs [43]. Cases of pregnancy termination before 20 weeks of gestation, fetal chromosomal abnormalities, fetal abnormalities related to twin-unique complications (eg, twin–twin transfusion syndrome, twin reversed arterial perfusion sequence) were excluded.

Twins are not predisposed to any specific type of congenital anomaly, although congenital heart disease is more prevalent in monochorionic twins, particularly those with TTTS [40]. In addition to an anatomic survey at 18 to 20 weeks of gestation, we perform fetal echocardiography at 18 to 20 weeks for monochorionic but not dichorionic twins because 2 to 10 percent of monochorionic twins referred for routine fetal echocardiography have congenital heart disease in at least one twin [44,45] and fetal echocardiography has a role in the diagnosis and management of TTTS. Although some studies have reported an increased risk of congenital heart disease in twins conceived by assisted reproductive technology, particularly IVF, compared with spontaneously conceived twins [46,47], others do not support this association [48-51]. We generally do not refer twin pregnancies for fetal echocardiography for the sole indication of conception via IVF, but some authorities consider IVF an indication for echocardiography in all twin pregnancies [52].

The reported accuracy of ultrasound for detection of fetal anomalies in twins varies because of differences in ascertainment postnatally and at pregnancy termination, criteria for defining an anomaly, and operator capability. Ultrasound examination can detect the majority of major malformations in twins when performed by sonographers experienced in both anomaly detection and assessment of multiple gestation. Sometimes more than one examination needs to be performed to adequately assess both twins.

Screening for short cervical length — At the second trimester fetal anatomy scan, we routinely evaluate cervical length by transabdominal ultrasound and then perform transvaginal ultrasound if the cervix is not well imaged or if it appears short. Others [53], including some UpToDate authors and editors, have taken a different approach: they measure cervical length by transvaginal ultrasound every two weeks between 16 and 24 weeks, as in singleton pregnancies. Practice patterns regarding cervical length measurement in twin pregnancies vary among clinicians (even among those working in the same medical facility), given the lack of high quality evidence favoring one approach over another [36]. (See "Short cervix before 24 weeks: Screening and management in singleton pregnancies".)

For patients with a short cervix, options include placement of a cerclage or treatment with daily vaginal progesterone. Management is discussed in detail separately. (See "Twin pregnancy: Management of pregnancy complications", section on 'Preterm labor and birth'.)

Screening for fetal growth restriction and discordance — Evaluation of fetal growth is particularly important in twin gestations because growth restriction and preterm birth are major causes of the higher morbidity/mortality rates in twin compared with singleton gestations [54-63]. However, as in singleton pregnancy, the performance of ultrasound examination for distinguishing the growth restricted fetus from the constitutionally small fetus is limited. (See "Fetal growth restriction: Screening and diagnosis".)

Normal growth — In the first and second trimesters, the growth rate of twins is not significantly different from that of singletons [64]. In the third trimester, particularly after 30 to 32 weeks of gestation, most studies have described slower fetal growth in uncomplicated twin gestations than in uncomplicated singleton gestations [64,65]. A prospective cohort study reported that almost 40 percent of dichorionic twins near term would be classified as small for gestational age if a singleton growth standard was used [65]. The slower growth rate has been attributed to anomalous umbilical cord insertion and to placental crowding (poor early development due to placental proximity).

Growth curves have been derived specifically for twins but are of limited usefulness since they were derived from relatively small populations and generally did not consider chorionicity and amnionicity or outcome. We and others believe that singleton growth curves are the best predictor of adverse pregnancy and developmental outcome in twin gestations and should be used for evaluating twins for growth abnormalities [66]. On the other hand, proponents of using twin growth curves state that singleton growth curves overdiagnose growth abnormalities, which can lead to unnecessary maternal anxiety, extra antenatal testing, and possibly iatrogenic preterm birth without improving neonatal outcome [67,68].

Approach to monitoring fetal growth — Serial ultrasound examinations are performed in the second and third trimesters to screen for growth restriction and growth discordance, given the risk of adverse outcome associated with these conditions and the insensitivity of symphysis-fundal height measurement for identifying fetal growth abnormalities in twins [69-71]. Although growth monitoring is typically performed in the second and third trimesters, discordance in crown-rump length may be observed as early as the first trimester and is predictive of later weight discordance. (See "Diagnosis and outcome of first-trimester growth delay", section on 'Twin pregnancies'.)

The growth of dichorionic twins who are both appropriate for gestational age is assessed every four to six weeks. In monochorionic twins, ultrasound examination is typically performed more frequently because they are also being monitored for development of TTTS, TAPS, and sFGR. (See "Twin-twin transfusion syndrome: Screening, prevalence, pathophysiology, and diagnosis" and "Selective fetal growth restriction in monochorionic twin pregnancies".)

Diagnosis of growth discordance and restriction — Estimated fetal weight and/or abdominal circumference can be used to monitor growth. (See "Prenatal assessment of gestational age, date of delivery, and fetal weight", section on 'Sonographic assessment of fetal weight'.) Growth abnormalities manifest in three ways:

●One twin can be small for gestational age (called selective fetal growth restriction)

●Both twins can be small for gestational age, or

●One twin can be significantly smaller than the other twin (ie, growth discordance) although neither is small for gestational age. In almost two-thirds of discordant twin pairs, the smaller twin has a birth weight <10th percentile [57].

There is no consensus regarding the optimum threshold for defining growth discordance in twins. Discordance in birth weight ranging from 15 to 40 percent has been considered predictive of adverse outcome [56,58-60,72-76]. We use an estimated weight difference ≥20 percent as the threshold for defining discordance, but ≥25 percent is also commonly used; both have low sensitivity (approximately 50 percent) but good specificity (approximately 90 percent) for predicting weight discordance at birth [77]. An intertwin abdominal circumference difference ≥20 mm, irrespective of gestational age is another threshold for defining growth discordance and has been reported to have 83 percent positive predictive value to detect a difference in birth weight ≥20 percent [78]. Discordance in other biometric measurements subsequently associated with adverse obstetric and neonatal outcome has also been described [79,80].

Despite routine use of ultrasound to monitor fetal growth, its ability to accurately identify discordant twins and adverse perinatal outcome is limited. In a systematic review, estimated fetal weight discordance ≥20 percent had sensitivity and specificity of 65 and 91 percent, respectively, for predicting birth weight discordance ≥20 percent [81].

There is no convincing evidence that Doppler velocimetry has benefits for detecting growth restriction over the use of ultrasound alone; therefore, routine use of Doppler velocimetry in twin gestations is not recommended [82,83]. However, Doppler ultrasound is useful for monitoring pregnancies in which the diagnosis of growth restriction, discordance, or fetal anemia has been made. (See "Selective fetal growth restriction in monochorionic twin pregnancies", section on 'Classification-based approach'.)

The management of growth restriction and/or discordance depends on chorionicity. (See "Twin pregnancy: Management of pregnancy complications", section on 'Growth restriction and discordance'.)

Screening for placental abnormalities — Sonographic evaluation of the placenta is particularly important in twin pregnancies because they are at increased risk of placental findings associated with adverse outcome. Identification and appropriate management of affected pregnancies can improve pregnancy outcome. These findings include:

●Placenta previa – The incidence of placenta previa in twin pregnancy is higher than in singletons (3.9 per 1000 live births versus 2.8 per 1000 live births) [84]. This has been attributed to the larger total area of twin placentas, especially dichorionic twin placentas. (See "Placenta previa: Epidemiology, clinical features, diagnosis, morbidity and mortality" and "Placenta previa: Management".)

●Vasa previa – The incidence of vasa previa in twin pregnancies is higher than in the general obstetric population (0.43 versus 0.04 percent [85]), consistent with the increased risk for placenta previa in twins [86,87]. Transvaginal examination with color Doppler mapping will detect vasa previa. (See "Velamentous umbilical cord insertion and vasa previa".)

●Velamentous cord insertion – Velamentous cord insertion, which is associated with an increased risk for fetal growth restriction and cord compression, has been reported in 12 percent of monochorionic twin pregnancies, 7 percent of dichorionic twin pregnancies, and 2 percent of singleton pregnancies [88]. (See "Velamentous umbilical cord insertion and vasa previa".)

PREGNANCY MONITORING BASED ON CHORIONICITY AND AMNIONICITY — Because monochorionic twin gestations are associated with greater and different risks than dichorionic twin gestations (table 3), protocols for monitoring monochorionic twins have involved more intensive surveillance than protocols for monitoring dichorionic twins, as described below.

Monochorionic/diamniotic twins — Monochorionic/diamniotic twins are at risk for developing TTTS, TAPS, and/or selective fetal growth restriction (sFGR). Early detection can improve perinatal outcome [89,90].

●The diagnosis of TTTS is based on the sonographic finding of oligohydramnios (maximal vertical pocket <2 cm) and polyhydramnios (maximal vertical pocket >8 cm). A specific protocol for monitoring (table 4) and management of affected pregnancies are described separately. (See "Twin-twin transfusion syndrome: Screening, prevalence, pathophysiology, and diagnosis" and "Twin-twin transfusion syndrome: Management and outcome".)

●The diagnosis of TAPS is based on MCA-PSV >1.5 multiples of the median (MoM) in one twin and <0.8 MoM in the other twin. A specific protocol for monitoring (table 4) and management of affected pregnancies are described separately. (See "Twin anemia-polycythemia sequence (TAPS)".)

●The diagnosis of sFGR in the second trimester is based on:

•Estimated fetal weight (EFW) <3^rd percentile of one fetus or

•At least two of the four following criteria:

-EFW <10^th percentile for one twin

-Abdominal circumference <10^th percentile for one twin

-Weight discordance ≥25 percent

-UA pulsatility index >95^th percentile for the smaller twin

Monochorionic placentation is a significant risk factor for discordant growth due to unequal sharing of the placenta or TTTS [91-95], although monochorionic placentation also appears to have a small independent adverse effect on intrauterine growth [96]. sFGR is different from the deficient uteroplacental perfusion that occurs in dichorionic twins and singletons. A specific protocol for monitoring (table 4) and management of affected pregnancies are described separately. (See "Selective fetal growth restriction in monochorionic twin pregnancies".)

Monochorionic/monoamniotic twins — Monochorionic/monoamniotic twins are at highest risk of adverse outcome because of the risk of cord entanglement; rarely, monoamniotic twins are conjoined. Monitoring and management of these pregnancies is reviewed separately. (See "Monoamniotic twin pregnancy (including conjoined twins)".)

Dichorionic twins — Dichorionic twins are at risk for growth restriction, but they are not at risk for TTTS and TAPS. A common practice is to perform an ultrasound examination every four to six weeks after 20 weeks of gestation to monitor fetal growth, as fetal growth deceleration leading to discordancy is optimally detected between 20 and 28 weeks of gestation [92]. Many twin fetuses with growth abnormalities can be identified by 20 to 24 weeks, so if there is no evidence of growth abnormality at that time, then repeated scanning might not be necessary [97]; however, we continue serial ultrasound assessment until birth. Routinely scanning every two to four weeks has been recommended by some authorities [53] and detected more abnormalities that prompted early delivery in one study, but whether this resulted in better perinatal outcomes was not determined [98].

ANTEPARTUM FETAL TESTING — Routine use of antepartum fetal testing (nonstress test [NST], biophysical profile [BPP], amniotic fluid volume determination, or Doppler velocimetry) has no proven benefit in uncomplicated twin pregnancies. However, antepartum fetal monitoring in twins is widely practiced beginning at approximately 32 weeks of gestation because of the increased risk of stillbirth in twins, particularly monochorionic twins [99]. The exact timing of initiation of testing varies among centers, depending on factors such as chorionicity and amnionicity and underlying complications.

In our practice, we routinely perform weekly testing in dichorionic twins beginning at 32 to 36 weeks, and in monochorionic diamniotic twin pregnancies beginning at 28 to 32 weeks of gestation; monochorionic monoamniotic twins are followed more closely (see "Monoamniotic twin pregnancy (including conjoined twins)"). If complications such as growth restriction develop, then testing is initiated earlier and/or performed more frequently. Either NSTs or BPPs can be used [100-102]. The best technique to assess amniotic fluid volume in diamniotic twin gestations is uncertain. Subjective assessment of the volume of amniotic fluid in each sac appears to be as accurate as quantitative assessment; however, the maximal vertical pocket (single deepest pocket) in each sac is commonly used. Using the amniotic fluid index (AFI), a four-quadrant technique used in singletons, is not appropriate in twin pregnancy. (See "Assessment of amniotic fluid volume", section on 'Multifetal pregnancy'.)

The American College of Obstetricians and Gynecologists suggests considering weekly antenatal fetal surveillance at 36+0 weeks of gestation for uncomplicated dichorionic twins, and individualizing surveillance of complicated dichorionic twin pregnancies [103]. For monochorionic twins, antenatal fetal surveillance is typically initiated at 32+0 weeks of gestation because of the increased risk of fetal death in these pregnancies.

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: Multiple gestation".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topics (see "Patient education: Having twins (The Basics)")

SUMMARY AND RECOMMENDATIONS

●Weight gain and nutrition: We follow National Academy of Medicine (formerly the Institute of Medicine) guidelines for cumulative weight gain by term for patients carrying twins (see 'Gestational weight gain' above):

•BMI 18.5 to 24.9 kg/m² (normal weight) – Weight gain 37 to 54 lb (16.8 to 24.5 kg).

•BMI 25.0 to 29.9 kg/m² (overweight) – Weight gain 31 to 50 lb (14.1 to 22.7 kg).

•BMI ≥30.0 kg/m² (obese) – Weight gain 25 to 42 lb (11.4 to 19.1 kg).

We also follow Society for Maternal-Fetal Medicine twin pregnancy caloric requirement and supplement recommendations, as shown in the table (table 1). (See 'Nutrition and supplements' above.)

●Preeclampsia prophylaxis: We suggest low-dose aspirin prophylaxis to reduce the risk of developing preeclampsia (Grade 2B). Patients with twin pregnancies are a high-risk group for developing preeclampsia. (See "Preeclampsia: Prevention", section on 'Low-dose aspirin'.)

●Physical activity: In early pregnancy, patients with uncomplicated twin pregnancies can generally follow the same exercise/physical activity recommendations as those with singleton pregnancies; however, the clinician should assess individual risk factors for preterm birth and counsel the patient accordingly. As pregnancy progresses, physical changes generally limit the duration and type of exercise performed. (See 'Physical and sexual activity, exercise' above.)

●Down syndrome screening: For women who choose to undergo screening for Down syndrome, we suggest either first-trimester combined test or noninvasive screening using cell-free DNA in maternal blood. While the combined test provides early, fetus-specific risk assessment because of inclusion of nuchal translucency measurement, cell-free fetal DNA has higher detection rates. Both fetuses should be karyotyped when karyotyping is performed since even monozygotic twins may be discordant. (See 'Screening for Down syndrome (trisomy 21)' above.)

●Screening for short cervical length: At the fetal anatomy scan at 18 to 22 weeks of gestation, we routinely evaluate cervical length by transabdominal ultrasound and then perform transvaginal ultrasound if the cervix is not well imaged or if it appears short. Others, including some UpToDate authors and editors, measure cervical length by transvaginal ultrasound every two weeks between 16 and 24 weeks, as in singleton pregnancies at high risk for preterm birth. Practice patterns regarding cervical length measurement in twin pregnancies vary among clinicians (even among those working in the same medical facility), given the lack of high quality evidence favoring one approach over another. (See 'Screening for short cervical length' above.)

●Screening for congenital anomalies: Each twin of a dizygotic pair has a similar congenital anomaly rate as a singleton and anomalies, if present, have a low concordance rate. The concordance rate of major congenital malformations in monozygotic twins is approximately 20 percent. In addition to a sonographic anatomic survey, we perform fetal echocardiography at 18 to 20 weeks for monochorionic but not dichorionic twins because 2 to 10 percent of monochorionic twins referred for routine fetal echocardiography have congenital heart disease in at least one twin and fetal echocardiography has a role in the diagnosis and management of twin-twin transfusion syndrome. (See 'Screening for congenital anomalies' above.)

●Monitoring fetal growth: Growth restriction is more common in twin than in singleton pregnancies and can manifest in three ways (see 'Screening for fetal growth restriction and discordance' above):

•One twin can be small for gestational age (called selective fetal growth restriction)

•Both twins can be small for gestational age, or

•One twin can be significantly smaller than the other twin (ie, growth discordance) although neither is small for gestational age. In almost two-thirds of discordant twin pairs, the smaller twin has a birth weight <10th percentile.

The growth of dichorionic twins who are both appropriate for gestational age is assessed every four to six weeks after 20 weeks of gestation. (See 'Dichorionic twins' above.)

Ultrasound examination is typically performed more frequently than monthly in monochorionic twins because they are also being monitored for development of twin-twin transfusion syndrome (TTTS), twin anemia-polycythemia sequence (TAPS), and selective fetal growth restriction (sFGR). (See 'Monochorionic/diamniotic twins' above.)

●Antepartum fetal testing: We perform weekly testing with nonstress tests and amniotic fluid evaluation or biophysical profile scoring starting at 32 weeks in almost all twin pregnancies (excluding monochorionic monoamniotic twins). Testing is performed earlier and/or more frequently if complications, such as fetal growth restriction, develop. (See 'Antepartum fetal testing' above.)