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Fetal echogenic bowel

Fetal echogenic bowel
Literature review current through: May 2024.
This topic last updated: Apr 18, 2024.

INTRODUCTION — The echogenicity of the fetal bowel is assessed during second-trimester obstetric ultrasound examinations because increased echogenicity is a marker for several fetal disorders, including some aneuploidies (most commonly trisomy 21, 13, and 18), cystic fibrosis (CF), some gastrointestinal abnormalities (eg, obstruction, atresia, perforation), growth restriction, and infection (most commonly cytomegalovirus [CMV]). Depending on the etiology, it may be associated with other fetal abnormalities and/or persist. When it is isolated and no other anomalies are seen, it can be a transient finding representing a normal variant.

This topic will discuss the diagnosis, possible etiologies, evaluation, and obstetric management of pregnancies with fetal echogenic bowel. Other types of fetal abdominal echogenicity are reviewed separately. (See "Fetal abdomen: Differential diagnosis of abnormal echogenicity and calcification".)

DEFINITION AND DIAGNOSIS

Definition – Fetal echogenic (or hyperechoic) bowel refers to increased echogenicity (brightness) of the fetal bowel on second-trimester ultrasound examination (image 1A-B). Echogenicity can be diffuse or focal, is uniform over a well-defined area that does not shadow, and is located primarily in the lower fetal abdomen and pelvis.

Diagnostic criteria – The sonographic criterion that we use is echogenicity similar to or greater than that of adjacent bone when the ultrasound gain is set to the lowest point at which bone appears white [1]. The iliac wing is the usual standard for comparison.

Some authorities have defined grades of echogenicity, with the most severe form (grade 3) as bright as bone, while grades 1 and 2 are mildly or moderately echogenic [2]. However, the prognostic significance of this classification is unclear [3].

Importance of transducer frequency – A limitation of the diagnostic criteria and proposed classification system is that echogenicity similar to or greater than that of adjacent bone is a subjective determination and therefore prone to interobserver and intraobserver variability [4]. Transducer frequency is an important factor because higher frequency transducers can make the differentiation between normal and abnormal bowel echogenicity difficult, leading to overdiagnosis of the latter. As an example, a study using both 8 and 5 MHz transducers sequentially on the same fetuses found the frequency of echogenic bowel was 31 and 3 percent, respectively [5]. For this reason, we only diagnose echogenic bowel when using a transducer frequency ≤5 MHz. Since transvaginal probes operate at higher frequencies than 5 MHz, a diagnosis of echogenic bowel should not be made based on the bowel appearance on transvaginal scanning alone.

DIFFERENTIAL DIAGNOSIS — Differential diagnosis involves exclusion of extraintestinal echogenic masses and calcifications in the fetal abdomen, including:

Gastric pseudomass, which appears as a clump of echogenicity in the stomach, most commonly in the second trimester from swallowed blood.

Meconium peritonitis, which appears as dilated loops of bowel and punctate calcifications. It is caused by bowel perforation; spillage of meconium into the peritoneum may be seen.

Meconium "pseudocyst", which appears as an echogenic mass outside of the bowel in fetuses with meconium peritonitis (image 2).

Detailed descriptions of these and other less common echogenic and calcified lesions in the abdomen can be found separately. (See "Fetal abdomen: Differential diagnosis of abnormal echogenicity and calcification".)

PREVALENCE ACROSS GESTATION

First trimester – Bowel echogenicity is not evaluable in the first trimester.

Second trimester – Prevalence ranges from 2 to 14 per 1000 singleton pregnancies examined in the second trimester, typically at the 18- to 20-week anomaly scan [6]. Increased echogenicity regresses or disappears during the remainder of the second trimester in up to three-quarters of these cases [7].

Third trimester – Echogenic bowel is generally a normal finding in the third trimester and reflects normal fetal physiology since meconium is present in the colon at this time and can be echogenic.

ETIOLOGY AND RELATED DISORDERS — The outcome of fetuses diagnosed with isolated echogenic bowel on antenatal ultrasound examination is shown in the table (table 1).

Overview — In over 80 to 90 percent of second-trimester fetuses with an isolated finding of echogenic bowel, no etiology is identified antepartum and the neonate (including the bowel) is normal [1,8,9]. This is particularly true in cases in which the echogenic bowel becomes less noticeable or disappears completely on subsequent ultrasound examinations [3]. However, approximately 2 percent of second-trimester fetuses with apparently isolated echogenic bowel after antepartum diagnostic evaluation will be found to have associated anomalies at birth, most commonly gastrointestinal abnormalities not detectable on antepartum ultrasound [7].

When structural abnormalities are also visualized (ie, nonisolated cases), an underlying aneuploidy or gastrointestinal anomaly is most likely [10,11].

Aneuploidy — Chromosomal anomalies were identified in 3.3 percent (95% CI 2.4-4.2) of fetuses with isolated echogenic bowel in a systematic review [7]. The most common aneuploidy was trisomy 21, which accounted for 39 of 50 fetuses (78 percent) with isolated echogenic bowel. Other aneuploidies and abnormalities that have been identified at low frequency include sex chromosome abnormalities, trisomy 18, trisomy 13, and 16p11.2 duplication.

Decreased bowel motility and increased water absorption have been reported in aneuploid newborns and may explain the pathogenesis of echogenic bowel in aneuploid fetuses [10,12].

Associated findings – The risk of aneuploidy is higher in fetuses who have structural anomalies (eg, cardiac anomalies, duodenal atresia) or who have multiple sonographic soft markers associated with aneuploidy. (See "Sonographic findings associated with fetal aneuploidy", section on 'Sonographic features of selected aneuploidies' and "Sonographic findings associated with fetal aneuploidy", section on 'Soft markers'.)

Blood in the bowel lumen — Placental and decidual bleeding can result in blood or blood components (pigments, cellular debris) in amniotic fluid. When ingested by the fetus, the bowel can appear echogenic since blood is highly echogenic (image 3) [13,14].

Associated findings – Ultrasound findings of a subchorionic fluid collection or echogenic material in the fetal stomach suggest intraamniotic bleeding or transmembrane passage of hemosiderin into amniotic fluid as the source of echogenic bowel. The mother may have a history of vaginal bleeding or an invasive obstetric procedure.

Although evidence of intraamniotic bleeding/transmembrane passage of hemosiderin into amniotic fluid suggests a possible etiology, such bleeding is not rare and does not exclude other etiologies of echogenic bowel.

Natural history – The natural history of this process was illustrated by a study of patients with intraamniotic bleeding after intrauterine fetal transfusion [13]. Bowel echogenicity developed in 7 of 28 fetuses within the first 12 hours after the bleeding episode and remained in five fetuses for at least two weeks after the procedure.

Cystic fibrosis — Approximately 2.2 percent (95% CI 1.5-3.0) of fetuses with isolated echogenic bowel had cystic fibrosis (CF) in a systematic review [7]. It is important to recognize that this autosomal recessive trait can be found in people of all ancestries, but is most common in non-Hispanic White individuals. Thus, the percentage of fetuses with echogenic bowel who have CF varies by parental ancestry.

In CF, abnormalities in pancreatic enzyme secretion result in thick, viscous meconium in the small bowel and hypoperistalsis, which is the likely reason for increased bowel echogenicity (image 4) [15].

Associated findings – The initial fetal sign of CF is echogenic bowel in the second trimester. If meconium ileus (terminal ileal obstruction from highly viscous impacted meconium) develops, then dilated loops of bowel, bowel perforation, meconium peritonitis, and meconium pseudocysts may be observed.

Fetal growth restriction — Approximately 13 percent (95% CI 6.1-21.1) of fetuses with echogenic bowel had fetal growth restriction (FGR) in a systematic review [7]. Echogenic bowel is more likely to be observed in fetuses at the lowest quartile for estimated weight [16].

It is possible that when fetal blood flow is redistributed toward vital organs (brain, heart, adrenals, placenta) in FGR, a subsequent reduction in blood flow to the gut results in the echogenic appearance of fetal bowel. However, there is no evidence supporting this hypothesis, and vasodilatation in the superior mesenteric artery and celiac trunk has been demonstrated in FGR [17,18].

Associated findings – The combination of echogenic fetal bowel and FGR, especially with oligohydramnios and elevated maternal serum alpha-fetoprotein (MSAFP) in the second trimester, has a poor prognosis [19-21]. Four to 8 percent of fetuses with both echogenic bowel and second-trimester FGR die in utero [9,19].

Infection — Congenital infection accounted for 2.2 percent (95% CI 1.0-3.7) of cases of isolated echogenic bowel in a systematic review [7]. Cytomegalovirus (CMV) is the most commonly detected fetal infection associated with isolated echogenic bowel, accounting for 10 of the 25 cases associated with infection in this review. Toxoplasmosis and parvovirus were less common. Varicella, rubella, and herpes simplex infection have also been described in association with echogenic bowel, but such cases are rare [14].

Congenital infection may cause echogenic bowel by direct damage or as a result of other potential sequelae of congenital infection, such as ascites or growth restriction. Ascites enhances the echogenicity of abdominal structures, such as bowel.

Associated findings – Ultrasound findings that have been associated with fetal infection include punctate calcifications within fetal organs (especially liver, brain, and/or eye), placenta, cord, and membranes; fetal anomalies (especially of the central nervous system); hydrops; and fetal growth restriction [22,23].

The overall sonographic findings of infections that have been associated with echogenic bowel are described in detail separately.

(See "Cytomegalovirus infection in pregnancy".)

(See "Toxoplasmosis and pregnancy".)

(See "Parvovirus B19 infection during pregnancy".)

(See "Varicella-zoster virus infection in pregnancy".)

(See "Genital herpes simplex virus infection and pregnancy".)

Gastrointestinal obstruction — Anomalies of the gastrointestinal tract at birth were identified in 1.1 percent of cases of echogenic bowel initially thought to be isolated in a systematic review [7]. Gastrointestinal anomalies that have been associated with echogenic bowel in case reports and small case series include bowel or anal atresia, volvulus, bowel duplication, gastroschisis, cloacal exstrophy, and biliary duct atresia [6,11,24,25]. There is at least one report of fetal echogenic bowel associated with Hirschsprung disease [26].

Gastrointestinal obstruction (anatomic or functional) may increase intestinal absorption of fluid from meconium within the obstructed bowel, and thereby increase bowel echogenicity [2,10]. However, the more typical sonographic findings associated with small bowel obstruction are dilated fluid-filled loops of bowel and polyhydramnios.

Associated findings – When the bowel is both echogenic and dilated, an underlying gastrointestinal obstructive anomaly is likely: 7 of 14 such cases required postnatal surgical intervention in one study [27] . An abdominal wall defect adjacent to the cord insertion site with free floating loops of bowel suggests gastroschisis. Cloacal exstrophy is multi-system congenital malformation involving anomalies of the genitourinary, gastrointestinal, musculoskeletal, and neurologic systems. In body stalk anomaly, an abdominal wall defect can be anywhere in the abdomen. (See "Fetal abdomen: Differential diagnosis of abnormal echogenicity and calcification" and "Body stalk anomaly and cloacal exstrophy: Prenatal diagnosis and management".)

Other etiologies

Echogenic bowel has been detected in fetuses with alpha-thalassemia [28]. The finding was attributed to bowel edema from severe anemia and hypoxemia.

In a large review, other conditions that have been observed in fetuses with echogenic bowel included cardiac, bone, central nervous system, or urinary tract anomalies and a variety of syndromes [11].

Most second-trimester cases of echogenicity are in the central abdomen and likely involve small bowel. Peripheral echogenicity is more likely to involve the colon and a common physiologic finding near term related to accumulation of meconium. Rarely, echogenicity restricted to the colon is associated with postnatal diagnosis of lysinuria-cystinuria, lysinuric protein intolerance, or hypotonia-cystinuria syndrome [29].

DIAGNOSTIC EVALUATION

History – Is there a recent history of suspected uterine bleeding? Echogenic bowel that develops soon after an episode of uterine bleeding, especially if associated with a gastric pseudomass, and resolves within two weeks is likely to be due to fetal swallowing of blood or hemosiderin in amniotic fluid. However, further diagnostic testing is reasonable since other causes of echogenic bowel cannot be excluded with certainty, particularly in the absence of a previously normal second-trimester sonogram. (See 'Blood in the bowel lumen' above.)

Detailed ultrasound examination – A detailed fetal ultrasound examination is required to determine whether echogenic bowel is an isolated finding or other abnormalities are present. This examination should be performed by an experienced obstetric sonographer or sonologist. We do not perform fetal echocardiography in all cases, but will do so if a cardiac abnormality is suspected.

Further evaluation depends on the findings of the detailed examination:

If associated findings are present and suggest a specific etiology, then the suspected etiology guides counseling and targeted diagnostic evaluation.

If the echogenic bowel appears to be isolated or is nonisolated without a suspected etiology, then the patient is counseled about the risk of an underlying fetal disorder and offered laboratory and aneuploidy testing, as described below.

Laboratory tests

Cystic fibrosis – We offer parental cystic fibrosis (CF) carrier screening if this has not been performed. To minimize the residual risk of CF in those without the most common pathogenic variants, we suggest obtaining the most extended panel of CF mutations available. If both parents are carriers, a prenatal diagnostic procedure for CF is offered. Testing to determine carrier status is reviewed separately. (See "Cystic fibrosis: Carrier screening".)

With advances in molecular technology, noninvasive fetal screening for CF using cell-free DNA from maternal plasma has become available commercially and has been described in the literature; however, the American College of Gynecologists (ACOG), the Society for Maternal-Fetal Medicine (SMFM), and the American College of Medical Genetics and Genomics (ACMG) have not recommended its use [30,31]. We offer this screening test in high-risk cases in patients who decline an invasive procedure for diagnostic testing. (See "Cystic fibrosis: Carrier screening", section on 'Noninvasive prenatal diagnosis'.)

CMV and toxoplasmosis – We order maternal serologies including immunoglobulin G (IgG), immunoglobulin M (IgM), and avidity testing for recent cytomegalovirus (CMV) or toxoplasmosis infection. In our opinion, the yield for other infections when increased echogenicity is isolated is too low to justify testing all pregnancies in which isolated echogenic bowel is detected [32].The yield for CMV is higher than for toxoplasmosis [33]. (See "Cytomegalovirus infection in pregnancy", section on 'Diagnosis of primary versus past maternal infection' and "Diagnostic testing for toxoplasmosis infection".)

If there is serologic evidence of recent maternal infection, fetal CMV and toxoplasmosis can be diagnosed by polymerase chain reaction of DNA in amniotic fluid or amniocytes. We generally await results of maternal serology before testing amniotic fluid. If amniocentesis is performed for other indications before serology results are available, the amniotic fluid can be held and tested at a later date, if indicated. (See 'Infection' above.)

Aneuploidy testing – The approach to diagnostic testing for fetal aneuploidy depends on whether other soft markers of aneuploidy are present, whether the patient has already had aneuploidy screening, the type of screening (serum biochemical markers with or without ultrasound measurement of nuchal translucency versus cell-free DNA), and the results.

For example, patients who have isolated echogenic bowel detected after cell-free DNA screening results indicating a low risk for trisomy 21 and 18 (ie, screen-negative) can be reassured of the extremely low possibility of aneuploidy but informed that the ultrasound finding is likely a normal variant or associated with a disorder other than aneuploidy (eg, CF or CMV infection).

For those classified as low risk of trisomy 21 and 18 based on less sensitive forms of aneuploidy screening (ie, serum biochemical markers with or without ultrasound rather than cell-free DNA), the degree of reassurance will depend on the calculated residual risk for these trisomies.

If aneuploidy screening was previously declined, the effect of the ultrasound finding on the a priori risk of aneuploidy is relatively small, but aneuploidy screening (serum biochemical markers or cell-free DNA) or diagnostic testing should be reoffered. Other scenarios (eg, multiple soft markers, different types of markers) are discussed in detail separately. (See "Prenatal genetic evaluation of the fetus with anomalies or soft markers", section on 'Approach to the evaluation of the fetus with "soft markers" and no structural anomalies'.)

POSTDIAGNOSTIC PRENATAL CARE — Routine prenatal care is appropriate for pregnancies with isolated echogenic bowel, defined as no evidence of fetal aneuploidy, cystic fibrosis (CF), growth restriction, structural anomalies, congenital infection, or comorbidities associated with an increased risk for fetal compromise (eg, preeclampsia). In several series of such patients, rates of perinatal morbidity and mortality, including long-term bowel pathology, have been similar to those in the general obstetric population [1,34,35].

The author’s practice is to perform a third-trimester ultrasound in pregnancies in which echogenic bowel was identified in the second trimester. The Society for Maternal-Fetal Medicine suggests an ultrasound examination at approximately 32 weeks of gestation to reevaluate fetal growth and the bowel [36].

POSTNATAL FOLLOW-UP — Pediatric providers should be informed of the antenatal finding of echogenic bowel and the results of the prenatal evaluation. There is no evidence to support additional postnatal evaluation other than physical examination of the newborn.

SUMMARY AND RECOMMENDATIONS

Definition and diagnosis – Fetal echogenic bowel refers to the second-trimester finding of bowel echogenicity similar to or greater than that of adjacent bone with the ultrasound gain set to the lowest point at which bone appears white using a 5 MHz or lower frequency transducer (image 1A-B). Hyperechogenicity can be diffuse or focal, is uniform over a well-defined area that does not shadow, and is located primarily in the lower fetal abdomen and pelvis. (See 'Definition and diagnosis' above.)

Prevalence – Fetal echogenic bowel is noted in up to 1 percent of second-trimester sonograms. It regresses or disappears during the remainder of the second trimester in most cases and is a normal finding in the third trimester since meconium is present in the colon at this time and can be echogenic. (See 'Prevalence across gestation' above.)

Etiology – In most cases, echogenic bowel is an isolated transient finding representing a normal variant. However, it can be associated with underlying pathology, most commonly swallowed blood or hemosiderin in amniotic fluid, aneuploidy, cystic fibrosis (CF), growth restriction, infection (especially cytomegalovirus), or a gastrointestinal malformation (table 1). (See 'Etiology and related disorders' above.)

Diagnostic evaluation – When fetal echogenic bowel is noted on second-trimester ultrasound examination, it is important to determine whether it is an isolated finding or if other anomalies are present, which may suggest an etiology. Patients should be counseled about potential fetal disorders and offered appropriate testing. We suggest the following approach (see 'Diagnostic evaluation' above):

An experienced obstetric sonographer or sonologist should perform a thorough ultrasound examination including a detailed fetal anatomic survey with attention to the gastrointestinal tract, heart and outflow tracts, estimation of fetal size/growth, assessment of markers suggestive of aneuploidy, and evaluation for signs of fetal infection.

We offer parental CF screening if not already performed and perform maternal serologic testing for cytomegalovirus (CMV) and toxoplasmosis infection.

We offer fetal genetic assessment using cell-free DNA (preferred) or serum screening for trisomy 21, 18, and 13 in maternal blood, if not already performed. We also offer invasive diagnostic testing to all patients as an alternative to screening.

If parental CF, aneuploidy, or infection screening yields positive results, we suggest amniocentesis to obtain a definitive fetal diagnosis.

Postdiagnostic prenatal care – Routine prenatal care is appropriate for pregnancies with isolated echogenic bowel and no evidence of fetal aneuploidy, CF, growth restriction, structural anomalies, or congenital infection. If fetal size and amniotic fluid are normal in the second trimester, we suggest an ultrasound examination at approximately 32 weeks to evaluate fetal growth and reevaluate the fetal bowel, since these fetuses are at increased risk of growth restriction and a gastrointestinal anomaly may not have been detected on earlier ultrasound examinations. (See 'Postdiagnostic prenatal care' above and 'Fetal growth restriction' above.)

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