Fetal Echogenic Bowel

Etiology

FEB was initially described as a normal variant and a transient finding during prenatal ultrasound. Most fetuses with an isolated echogenic bowel have normal short- and long-term neonatal outcomes, and the majority show regression of this finding in subsequent scans.[7][8][9] 

However, echogenic bowel can be associated with chromosomal abnormalities (eg, trisomies 21, 13, 18), cystic fibrosis, congenital infection (eg, TORCH infections), intrauterine growth restriction, gastrointestinal malformations (eg, gastric obstruction, atresia, perforation, meconium ileus), placental insufficiency, intraamniotic bleeding as well as intrauterine and neonatal death.[4][10][11][12] The incidence of these associations varies considerably between studies.[13]

An echogenic bowel is more closely associated with multi-system malformations in fetuses with underlying chromosomal abnormalities. The most common associated systemic involvement in these cases is cardiovascular anomalies (eg, tetralogy of Fallot, coarctation of the aorta, pulmonary artery hypoplasia, tricuspid atresia, cardiomegaly), urinary tract abnormalities (eg, posterior urethral valves, megabladder, bilateral renal dysplasia), craniocerebral anomalies (eg, hydrocephalus), and gastrointestinal malformations.[4]

Isolated echogenic bowel has been reported in a few cases with the use of natural iron-rich mineral water during pregnancy.[14] Additionally, other studies have noted differences in intestinal microbiota after delivery between healthy fetuses and those with hyperechogenic bowel.[15] However, these associations still require further investigation.

Epidemiology

The incidence of echogenic bowel varies from 0.2% to 1.8% in prenatal sonograms performed in the second trimester.[16] The variable incidence between different studies could be attributed to the high-risk population in some samples. Additionally, this could also be attributed to the fact that the detection of echogenic bowel is prone to significant inter-observer variability since the diagnostic criteria are based on a subjective assessment, which is prone to over- or underestimation of the finding based on the level of expertise of the sonographer as well as the frequency and the gain settings of the ultrasound machine.[3][17]

Pathophysiology

The pathophysiology of FEB is related to the underlying etiology. The fetus begins to swallow amniotic fluid by the end of the first trimester, which transits through the intestines. When this passage is slow or obstructed, the contents in the bowel can become progressively thick, leading to a bright appearance during the ultrasound. The primary mechanism may involve meconium stasis or hypercellular meconium, which can occur for several reasons.[17] 

Meconium Stasis

Meconium stasis can occur reasons, include the following:

• Hypoperistalsis
• Distal obstruction of the bowel 
• Meconium ileus
• Fetal intestinal ischemia
• Fetal intestinal perforation

Infants with abnormal karyotypes may have decreased levels of microvillar enzymes, leading to hypoperistalsis, further impeding the passage of meconium in utero. This meconium can be inspissated, especially in the second trimester when the bowel lumens are small. As the pregnancy progresses, the amount of fetal swallowing of amniotic fluid increases, along with the volume of the small bowel, resulting in the resolution of meconium stasis and, consequently, the resolution of the hyperechogenic appearance of the bowel.[18]

The pathophysiology of the association between echogenic bowel and TORCH infection is also thought to result from direct viral-induced cytotoxic effect and ischemia of the endothelial cells in the fetal bowel, leading to hypoperistalsis, ileus, or fetal intestinal perforation.[19] Proximal bowel obstruction can interfere with normal amniotic fluid swallowing and lead to decreased fluid content in the meconium, which subsequently leads to the formation of meconium plugs, which can be identified as increased echogenicity.[20]

Hypercellular Meconium

Hypercellular meconium can occur for the following reasons:

• Abnormal enzyme secretion
• Proximal obstruction of the bowel
• Feto-placental bleeding into the amniotic fluid
• Intrauterine transfusions

Conditions such as cystic fibrosis can lead to abnormalities in enzyme secretion, resulting in the abnormal consistency of meconium. This is usually detected as diffusely increased echogenicity of the bowel along with bowel dilatation and calcification.[21] Swallowed blood products due to maternal-fetal hemorrhage or intrauterine transfusions can also lead to hypercellular meconium and the finding of echogenic bowel.[20] Other possibilities, such as compromised placental and fetal perfusion leading to the redistribution of blood flow and, subsequently, gut ischemia, have been postulated.[22]

History and Physical

History and physical examination can help differentiate various pathologies associated with FEB. Prenatal history should focus on additional pregnancy risk factors (including advanced maternal age, gestational diabetes, fetal growth restriction, and polyhydramnios), other major and minor fetal anomalies, results of prior imaging and screening tests, past medical history, and comorbidities.[16] The postnatal examination should be comprehensive, with special attention to congenital anomalies. A thorough abdominal exam can provide clues, such as palpable intestinal loops indicating bowel obstruction. The patency of the neonatal gastrointestinal tract should also be confirmed.

Evaluation

A pragmatic approach should be adopted while investigating pregnancies with the finding of FEB in the second trimester.[11] Patients should be stratified by FEB grade and the presence of any additional risk factors or findings. Further evaluation should be offered, and an interdisciplinary team, including a genetic counselor, neonatologist, and maternal-fetal medicine (MFM) specialist, should be involved. 

Comprehensive Ultrasound Evaluation

A detailed ultrasound evaluation should be performed to assess for additional markers or features suggesting chromosome abnormalities (eg, nuchal translucency, cardiac anomalies).[6] If there is a normal 4-chamber view and the outflow tracts are visualized as normal, then a fetal echocardiogram is not necessary. However, if these views are abnormal or not obtainable, a fetal echocardiogram should be performed.[23] Assessment of amniotic fluid is also helpful in detecting maternal-fetal hemorrhage as a cause of the echogenic bowel. Fetal magnetic resonance imaging (MRI) could also be considered in cases of FEB associated with bowel dilatation.[13] 

Genetic Screening Recommendations

The Society for Maternal-Fetal Medicine (SMFM) recommends counseling to estimate the probability of aneuploidy, and, for patients without previous aneuploidy screening, further genetic screening, including cell-free fetal DNA testing or a quadruple screen (if cell-free DNA is unavailable or cost-prohibitive).[6][24]

Invasive Diagnostic Testing

Amniocentesis should be offered to all patients, regardless of screening results. In cases with additional findings, including the presence of other soft sonographic markers, major anomalies, or a positive aneuploidy screen, amniocentesis should be offered for definitive diagnosis, and chromosomal microarray can be considered.[9] Additionally, if amniocentesis is performed in patients with FEB, amniotic fluid should be sent for fetal karyotype and polymerase chain reaction for congenital infections.

It is also important to note that echogenic bowel is present as an isolated finding in 4% to 25% of fetuses with aneuploidy, making invasive diagnostic testing reasonable to offer all patients with FEB.[6][25][26] However, SMFM recommends no additional testing for aneuploidy specifically due to an isolated FEB with negative genetic screening results.[6] 

Cystic Fibrosis Screening

Cystic fibrosis screening should be performed if it has not already been done as part of routine screening. If both parents are found to be carriers, the patient should be referred for genetic counseling, and diagnostic testing should be offered.[6]

Serologic Testing for TORCH Infections

A thorough history should be performed to screen patients for maternal infection with potential causative agents, including cytomegalovirus (CMV), toxoplasmosis, rubella, varicella, herpes, and parvovirus. Routine testing for other potential congenital infections is generally not useful, as the chance of a positive result is low without a history of exposure or other clinical risk factors.[6]

Although symptomatic maternal infection is uncommon, CMV immunoglobulin G (IgG) and IgM titers should be ordered, with IgG avidity testing as needed. If results suggest primary CMV infection (eg, positive IgM with low avidity IgG), amniocentesis with polymerase chain reaction (PCR) for CMV should be considered to confirm the diagnosis.[6][27]

Treatment / Management

Diagnosis of echogenic bowel in the second trimester warrants thorough antenatal and postnatal follow-up due to the increased risk of fetal abnormalities. (see Image. Diagnostic Flowchart of a Fetus With Hyperechogenic Bowel.) The management of FEB depends on the degree of echogenicity and the presence of associated findings. Early identification and appropriate follow-up can help optimize outcomes for both the fetus and the expectant parents.

Prenatal Management

Serial sonographic screening throughout the pregnancy is recommended for reassessment and to monitor for associated conditions, including fetal growth restriction, bowel dilatation, and polyhydramnios, which are themselves associated with adverse outcomes.[20] SMFM recommends a third-trimester ultrasound for reassessment of FEB and evaluation of fetal growth.[6]

A large retrospective cohort study of singleton gestations found that, after excluding cases of echogenic bowel due to aneuploidy and CMV infection, the risk of intrauterine fetal demise was 7.3% in the group with isolated echogenic bowel on mid-trimester ultrasound compared to 0.9% in the nonechogenic bowel group.[28] In this study, all fetal deaths in the echogenic bowel group occurred before 30 weeks, at an average gestation of 23.7 weeks. For this reason, the authors suggest that early initiation of serial ultrasounds may be warranted. (B2)

Postnatal Management

The association of gastrointestinal abnormalities with other comorbidities in infants with a diagnosis of FEB highlights the need for these children to be delivered at centers with higher-level neonatal intensive care capabilities and pediatric subspecialists readily available. Pediatric care providers attending the birth should be informed of the antenatal findings of FEB to ensure appropriate postnatal evaluation.

Postnatally, a comprehensive clinical examination must be performed, with particular attention to the abdomen to detect palpable loops of bowel, which could indicate potential obstruction. Passage of meconium and normal feeding patterns should be ensured. Any delay in meconium passage beyond 24 to 48 hours of life or decreased stool output during the first week of life may indicate meconium ileus and should prompt additional consideration for cystic fibrosis while awaiting newborn screening results. Careful monitoring is indicated to prevent complications.

Abdominal x-rays, upper gastrointestinal series, barium, or gastrograffin enema may be required to evaluate for intestinal obstruction.[29] When meconium ileus is suspected, water-soluble contrast enemas can be both diagnostic and therapeutic.(B3)

Genetic and infectious disease evaluation should be pursued based on prenatal risk stratification. If it is an isolated finding, and cystic fibrosis and chromosomal conditions have been excluded, there is no need for postnatal follow-up or any further treatment. However, when antenatal testing is incomplete or suggests abnormalities, additional testing for congenital infections may be warranted based on clinical presentation and prenatal findings. For infants requiring surgical intervention or intensive monitoring, a multidisciplinary team approach involving neonatologists, pediatric surgeons, and subspecialists in genetics and pulmonology ensures optimal outcomes.

Differential Diagnosis

The prenatal ultrasound finding of echogenic bowel is considered a soft marker, and when detected in the second trimester, it should prompt further evaluation to detect additional abnormalities. As discussed above, it may be associated with multiple etiologies. The following differential diagnoses should be considered while evaluating FEB.

• Increased cellularity or thick meconium: intestinal obstruction, intestinal atresia, cystic fibrosis, chromosomal abnormality
• Fetal swallowing of blood: fetoplacental hemorrhage, bleeding into the amniotic fluid, placental abruption
• Inflammation of the bowel: TORCH infection
• Decreased blood flow in the bowel: fetal growth restriction, feto-placental insufficiency
• False-positive finding: frequency and gain settings of the ultrasound machine altered images, expertise of the sonographer

The diagnosis can often be clarified by identifying additional clues that suggest the underlying etiology. For example, additional abnormalities, such as a congenital heart defect or a thick nuchal cord, increase the likelihood of an aneuploidy. Features of fetal ascites or hydrops are more suggestive of an infectious etiology, whereas a thickened, dilated bowel wall associated with signs of obstruction or calcification may indicate cystic fibrosis.[29]

Prognosis

The prognosis of FEB depends upon the underlying pathophysiology.[30] Isolated echogenic bowel can be a benign finding with normal neonatal outcomes. However, outcomes can be impacted when it is associated with other ultrasonographic markers or pregnancy complications. Pregnancies complicated by FEB and associated with other anomalies, fetal growth restriction, elevated alpha-fetoprotein, or progressive worsening in grading are associated with less favorable to poor outcomes.[16]

Complications

Complications associated with echogenic bowel are related to the underlying pathophysiology. In pregnancies associated with intrauterine growth restriction or elevated alpha-fetoprotein, complications may arise from underlying feto-placental bleeding or congenital anomalies. The association between echogenic bowel and intrauterine fetal death may be related to underlying uteroplacental insufficiency.[31]