Fetal Abdominal Imaging

Definition

Ascites is an abnormal fluid collection within the fetal peritoneal cavity and is often the first finding in hydrops fetalis. Ascites is present in 85% of cases of nonimmune hydrops fetalis. Isolated fetal ascites indicates there is fluid accumulation in the abdominal cavity without involvement of other body cavities or subcutaneous edema.

Incidence and Epidemiology

• Hydrops fetalis occurs in approximately 1:1700–3000 pregnancies ; the incidence of isolated ascites is much lower.

Etiology and Pathogenesis

• Free fluid in the abdomen could represent

  • A transudate, owing to increased fluid leaking from the fetal capillary beds without adequate lymphatic return.

  • An exudate, as an inflammatory response to infection or malignancy.

  • Leaked urine, from an overdistended or ruptured bladder.

  • Meconium, from perforated bowel producing meconium peritonitis.

  • Chylous fluid, due to peritoneal lymphangiectasia (Waldmann disease) or lymphatic hypoplasia.

• Common etiologies for isolated fetal ascites are listed in Box 24.1 .

  Box 24.1

  Common Anomalies Associated With Isolated Fetal Ascites

  Data from Zelop C, Benacerraf BR. The causes and natural history of fetal ascites. Prenat Diagn . 1994;14:941.

  Gastrointestinal

  • Chylous lymphangiectasia

  • Meconium peritonitis

  • Cystic fibrosis

  • Biliary atresia

  Cardiac

  • Fetal arrhythmia

  • Congenital heart defects

  Genitourinary

  • Ruptured bladder or kidney

  • Bladder outlet obstruction

  • Ruptured fetal ovarian cyst

  Infection

  • TORCH infections, particularly CMV

  • Parvovirus B19

  • Syphilis

  Isoimmunization

  • Rh incompatibility

  • Atypical red cell antigen incompatibility (e.g., Kell isoimmunization)

  Miscellaneous

  • Nonimmune hydrops fetalis

  • Fetal-maternal hemorrhage

  • Inborn errors of metabolism

  • Aneuploidy

  • Fetal neoplasia

Diagnostic Features

• Ascites appears as an echolucent fluid accumulation within the fetal abdomen, outlining the fetal liver, loops of bowel, stomach, and bladder ( Figs. 24.1–24.3 ; Video 24.1 , Video 24.2 , Video 24.3 , Video 24.4 , Video 24.5 ).

• In extreme cases of ascites, the falciform ligament and/or extrahepatic portion of the umbilical vein may be outlined by fluid ( Fig. 24.4 ).

• Cases of meconium peritonitis exhibit echogenic calcifications on peritoneal surfaces.

• Systematic assessment of fetal anatomy ^, should be undertaken for other abnormalities and for evidence of hydrops fetalis, such as

  • Scalp and skin edema ( Video 4.6).

  • Pleural effusion

  • Pericardial effusion

  • Polyhydramnios

  • Placentomegaly (>4 cm thick)

Differential Diagnosis

• Markedly overdistended bladder (megacystis) as seen with posterior urethral valves

• Ovarian cyst

• Dilated fetal bowel, particularly descending and sigmoid colon

• Pseudoascites

  • Thin echolucent rim produced by abdominal wall muscles can be confused for ascites ( Fig. 24.5 ).

  • Acoustic “shadowing” from echodense structures (e.g., bone) proximal in the ultrasound beam can mimic ascites ( Fig. 24.6 ).

Associated Anomalies

• Gastrointestinal abnormalities (e.g., meconium peritonitis) more likely to be associated with late diagnosis of fetal ascites (3% versus 30% if detected >24 weeks; P = .016)

• Hydronephrosis, megacystis

• Echogenic and/or dilated bowel

• Hepatomegaly

• Fetal growth restriction

• Polyhydramnios more common (39%) than oligohydramnios (16%)

Imaging

Prognosis

• Overall prognosis depends on underlying condition and/or structural abnormalities.

  • Best prognosis with chylous ascites (0% mortality)

  • Poor prognosis if

    • Ascites diagnosed at <24 weeks (32% mortality versus 0% mortality if detected >24 weeks; P = .003)

    • Associated fetal hydrops (approximately 75% mortality)

  • Worst prognosis with metabolic storage disease (100% mortality)

• 30%–75% of cases of isolated fetal ascites resolve spontaneously. ^{, ,}

Antenatal Management

• Detailed ultrasound examinations to detect associated fetal anomalies.

• Doppler assessment of umbilical and middle cerebral artery flow.

• Fetal echocardiogram to confirm normal anatomy.

• Maternal blood testing including

  • Blood type and antibody screen

  • Kleihauer-Betke testing to detect fetal hemoglobin in maternal circulation (fetomaternal hemorrhage)

  • Rapid plasmin reagin (RPR)

  • TORCH ( t oxoplasmosis, o ther agents, r ubella, c ytomegalovirus, and h erpes simplex), Listeria, and parvovirus antibody titers

  • Genetic disease carrier screening

• Amniocentesis with microarray testing or noninvasive prenatal testing (NIPT) to assess

  • Karyotype

  • Infection (polymerase chain reaction [PCR] for cytomegalovirus [CMV] and parvovirus)

  • Genetic or metabolic disease

• Cordocentesis to assess hemoglobin, Coombs test, and infection.

• Consider diagnostic paracentesis for protein concentration, triglyceride, cultures, and lymphocyte count to determine transudate versus exudate; low protein concentration, negative cultures, and lymphocyte count >90% suggest fetal chylous ascites.

• Serial ultrasound examinations every 1–2 weeks to monitor fetal growth, amniotic fluid, worsening ascites, and progression to generalized hydrops fetalis.

• Begin fetal nonstress test and/or biophysical profile testing at viability.

• Delivery in tertiary care facility is recommended.

• Consider therapeutic fetal paracentesis prior to delivery.

  • Prevents abdominal dystocia and facilitates vaginal delivery

  • Decreases mechanical compression of lungs

  • Facilitates neonatal resuscitative efforts

• Cesarean delivery should be reserved for obstetric indications.

Neonatal Management

• Neonatal therapeutic paracentesis should be considered if ascites causing respiratory compromise.

• Continue diagnostic evaluation to reveal underlying etiology of ascites.

  • Abdominal ultrasound

  • Paracentesis with cell count, culture, and fluid analysis

    • Chylous ascites typically will have

      • High cell count (predominantly lymphocytes)

      • Total protein 2.5–7.0 g/dL

      • Serum to ascites albumin gradient (SAAG) <1.1 g/dL

      • High triglyceride (>200 g/dL), low-cholesterol

      • Low glucose (<100 mg/dL)

• • Consider lymphangiography

• Total parental nutrition (TPN) and/or medium chain triglyceride-based formula (e.g., monogen feeds) reduce chyle volume. ^,

• Periodic therapeutic paracentesis may be warranted to improve respiratory function.

• Surgical management if meconium peritonitis or malrotation suspected.

Key Points

• Ascites may be the first finding in hydrops fetalis and should prompt a systematic fetal assessment to rule out this condition.

• The patient should be thoroughly evaluated to rule out fetal anemia, infection, and other etiologies of hydrops fetalis.

• Continued surveillance is important to rule out evolving hydrops fetalis; however, many cases of isolated fetal ascites resolve spontaneously.

Definition

Intestinal atresia represents complete obstruction of the bowel lumen; stenosis is a narrowing of the lumen of the fetal bowel. Both can produce a distended proximal gastrointestinal tract.

Incidence and Epidemiology

• Intestinal atresias occur in approximately 1–3:10,000 live births.

• Duodenal atresia prevalence varies from 1:5000 to 1:10,000.

• Jejunoileal atresia occurs in 1:3000.

• Colonic atresia is seen in 1:2000 to 1:5000.

• Anal atresia is present in 1:5000.

• Up to one-third have multiple atretic sites.

Etiology and Pathogenesis

• The cause of intestinal atresias varies by location of the obstruction.

  • Duodenal atresia —lack of revacuolization during the solid core stage of embryonic intestinal development

  • Jejunoileal and colonic atresias —vascular accident, intestinal volvulus, intussusception, internal hernia, or strangulation as in the case of tight abdominal wall defect with gastroschisis or omphalocele

Diagnostic Features

• Bowel loops >7 mm are suggestive of obstruction; normal colon and rectal diameter progressively enlarges with advancing gestational age ( Table 24.1 ).

  Table 24.1

  Fetal Colon and Rectal Diameters

  Data from Zalel Y et al. In-utero development of the fetal colon and rectum: Sonographic evaluation. Ultrasound Obstet Gynecol. 2003;21:161-4.

  	Descending Colon Diameter (mm)		Rectal Diameter (mm)
  Gestational Age, Weeks	Mean	95% CI	Mean	95% CI
  19-20	3.52	0.79–6.26	3.64	1.45–5.82
  21	3.59	0.86–6.32	3.79	1.61–5.97
  22	3.69	0.96–6.41	3.95	1.78–6.13
  23	3.82	1.09–6.54	4.14	1.97–6.31
  24	3.98	1.26–6.70	4.34	2.17–6.52
  25	4.18	1.46–6.90	4.57	2.40–6.74
  26	4.43	1.70–7.15	4.82	2.64–6.99
  27	4.71	1.99–7.43	5.08	2.91–7.26
  28	5.04	2.32–7.76	5.38	3.20–7.55
  29	5.42	2.69–8.14	5.69	3.52–7.87
  30	5.84	3.12–8.57	6.04	3.86–8.21
  31	6.32	3.60–9.05	6.41	4.23–8.58
  32	6.86	4.13–9.58	6.80	4.63–8.98
  33	7.45	4.72–10.17	7.23	5.05–9.40
  34	8.10	5.37–10.82	7.68	5.51–9.85
  35	8.81	6.09–11.53	8.17	5.99–10.34
  36	9.59	6.87–12.31	8.68	6.51–10.85
  37	10.44	7.71–13.16	9.23	7.16–11.40
  38	11.35	8.63–14.08	9.81	7.64–11.98
  39	12.34	9.61–15.07	10.43	8.25–12.61
  40	13.40	10.66–16.15	11.08	8.89–13.26

• Dilated bowel and/or colon may not be evident until the third trimester.

• Duodenal atresia produces classic echolucent “double bubble.”

  • Represents two fluid-filled structures: the stomach and dilated proximal fluid-filled duodenum ( Fig. 24.7 ; Video 24.7 )

  • A thin echolucent connection between these two echolucent structures, most commonly visible after the late second trimester, confirms the diagnosis of duodenal atresia ( Fig. 24.8 ; Video 24.8 , Video 24.9 ).

  • Polyhydramnios is typical with duodenal atresia and is particularly marked in the third trimester.

• Distal intestinal lesions are difficult to diagnose prenatally; many cases of jejunoileal atresia may remain undetected before delivery.

  • Detection rate of nonduodenal small bowel atresia is approximately 50% (95% confidence interval [CI], 38.0%–63.2%), with only 25% of ileal atresia identified in utero (95% CI, 4.0%–58.0%).

  • Jejunoileal atresia produces multiple dilated loops of bowel with increased proximal peristalsis.

  • Multiple dilated bowel loops likely indicate jejunal obstruction ( Fig. 24.9 ), whereas smaller multiple dilated loops of bowel may indicate more distal ileal obstruction ( Fig. 24.10 ; Video 24.10 , Video 24.11 ).

• Colon obstruction may be challenging to diagnose prenatally. ^,

  • Isoechoic meconium fills the dilated colon, making echolucency less apparent.

  • Colonic atresia primarily occurs proximal to splenic flexure with distal microcolon, which is not visible on prenatal ultrasound.

  • Late in pregnancy, the colon can become markedly dilated ( Figs. 24.11–24.13 ).

  • A U- or V-shaped segment of dilated echolucent bowel is suggestive of anorectal atresia ( Fig. 24.14 ).

  • Imperforate anus will result in a dilated rectum with echogenic meconium contents ( Fig. 24.15 ; Video 24.12 ).

  • Dilated colon containing echogenic material may represent a cloacal anomaly or rectourinary fistula ( Fig. 24.16 ).

• Careful sonographic anatomic survey is needed to detect other anomalies.

Differential Diagnosis

• Mildly dilated, normal colon

• Markedly distended normal fetal stomach or gallbladder

• External constriction by annular pancreas or Ladd bands encircling duodenum

• Meconium ileus, particularly associated with cystic fibrosis

• Hirschsprung disease (aganglionosis primarily affecting distal colon and rectum)

• Intestinal malrotation

• Congenital chloride diarrhea

• Hydroureter

• Hydrocolpos

• Umbilical vein varix

• Other abdominal cystic masses that can mimic distended bowel include

  • Choledochal cyst

  • Ovarian cyst

  • Duplication cyst

  • Mesenteric cyst

  • Renal cyst

  • Splenic cyst

  • Urachal cyst

Associated Anomalies

• Nearly half of intestinal atresia cases have an associated abnormality.

• Trisomy 21 may be present in 30% of duodenal atresia cases.

• Congenital heart defects (e.g., atrial and ventricular septal defects).

• Gastrointestinal malformations

  • Malrotation

  • Meconium ileus and peritonitis

  • Volvulus

  • Annular pancreas (seen in 20%–30% of duodenal atresia cases)

  • Esophageal atresia

  • Anorectal atresia

• Genitourinary malformations.

• Ventral wall defects.

• Skeletal defects, primarily with anal atresia (VATER syndrome [ v ertebral defects, imperforate a nus, t racheo e sophageal fistula, r enal defects], VACTERL association [ v ertebral abnormality, a nal atresia, c ardiac defect, t racheo e sophageal fistula, r enal agenesis, and radial l imb abnormality], OEIS complex [ o mphalocele, e xstrophy of the bladder, i mperforate anus, s pinal deformities], caudal regression).

Imaging

Prognosis

• Overall prognosis depends on level of lesion and other associated anomalies, particularly aneuploidy.

• Live birth rate is lower in duodenal atresia compared with ileojejunal atresia, owing to higher rate of associated congenital anomalies.

• Survival data have shown improvement over time from approximately 60% in the 1960s and 1970s to 90%–100% in the 1990s and 2000s.

• Jejunoileal atresia has consistently lower long-term survival (60%–90%), secondary to short bowel syndrome.

• Long-term total parenteral nutrition (TPN) may lead to cholestasis and subsequent liver damage.

• Small bowel transplantation may be necessary; 5-year patient survival rate is 55%; graft survival rate is 45%.

Antenatal Management

• Amniocentesis with microarray or noninvasive prenatal testing (NIPT) to assess karyotype and cystic fibrosis status.

• Fetal echocardiogram.

• Consider MRI to determine level of obstruction. ^,

• Monitor fetal growth, amniotic fluid, worsening bowel distention, and progression to bowel perforation with meconium peritonitis.

• Fetal nonstress and/or biophysical profile testing in third trimester.

• Pediatric surgery consultation.

• Delivery in tertiary care facility is recommended.

• Therapeutic amnioreduction may reduce risk of preterm labor and prolapsed umbilical cord or abruption with rupture of membranes.

• Cesarean delivery should be reserved for obstetric indications.

Neonatal Management

• Orogastric or nasogastric decompression to prevent aspiration

• IV fluid hydration

• Confirm diagnosis of intestinal atresia

  • Plain abdominal radiograph

  • Upper gastrointestinal contrast-enhanced radiograph

  • Barium enema

• Surgical management depends on site of lesion and associated conditions; most are treated with excision of abnormal segment and primary anastomosis :

  • Duodenal atresia is treated with direct duodenojejunostomy or duodenoduodenostomy.

  • Jejunoileal atresia is managed with tapering enteroplasty or intestinal plication.

  • Colonic atresia is repaired with primary anastomosis.

  • Multiple intestinal atresias are managed with multiple staged anastomoses and enteroplasty; approximately 25% require >100 days on TPN.

Key Points

• Dilated loops of fetal bowel may indicate obstruction distal to dilated segment of bowel or colon.

• Proximal small bowel (duodenal) atresia is more easily identified on second-trimester and third-trimester fetal imaging than more distal (jejunal or colonic) atresia.

• Marked polyhydramnios is typical in third trimester with proximal small bowel atresia and may result in preterm labor, premature rupture of membranes, and prolapsed umbilical cord.

Definition

Gastroschisis is a full-thickness paraumbilical defect through which bowel herniates. In contradistinction to omphalocele, the bowels are not enveloped by a membrane.

Incidence and Epidemiology

• The incidence of gastroschisis worldwide is approximately 1–5:10,000 live births. Although the incidence in US increased threefold between 1997 and 2008 (2.9–6.4:10,000), it decreased to 3.3:10,000 by 2018 ( P < .01).

• Risk factors for developing gastroschisis include young maternal age, smoking, vasoactive drug use, and low socioeconomic status.

  • Women younger than 25 years old have a two to three times higher risk of gastroschisis, with prevalence of gastroschisis in women younger than age 20 increasing from 8:10,000 to 18:10,000 since the mid-1990s.

  • Smoking increases risk of gastroschisis (odds ratio [OR] = 3.4; 95% CI, 1.1–10.5).

  • Illicit drug use (relative risk [RR] 2.14; 95% CI, 1.48–3.07) and alcohol consumption (RR 1.40; 95% CI, 1.13–1.70) increase the risk of gastroschisis.

  • Gastroschisis is increased approximately 1.5 times in areas where opioid prescription rates are high (5.1:10,000 live births; CI, 4.9–5.3) compared to low opioid prescription areas (3.2:10,000 live births; CI, 3.1–3.4).

  • Black women have a lower risk of gastroschisis (RR = 0.49; 95% CI, 0.38–0.63).

Etiology and Pathogenesis

• The embryogenesis of gastroschisis is unclear. There are five main proposed theories leading to gut herniation and clinical presentation of gastroschisis:

  • Embryonic (somatopleural) mesenchyme fails to differentiate; thus abdominal wall mesoderm fails to form.

  • Amnion rupture at base of the umbilical ring during physiologic gut herniation in late embryonic period.

  • Right umbilical vein prematurely involutes.

  • Right vitelline (omphalomesenteric) artery disrupted.

  • Abnormal body wall folding.

• Multifactorial etiology of gastroschisis includes identified gene variants ( NOS3, NPPA, ADD1, ICAM1, ICAM4, ICAM5, and MTHFR ) associated with environmental exposures.

Diagnostic Features

• Multiple loops of bowel are seen floating freely in amniotic fluid, with a typical “cauliflower” appearance ( Figs. 24.17 and 24.18 ; Video 24.13 , Video 24.14 , Video 24.15 ).

• Abdominal wall defect is typically located to the right of the abdominal cord insertion ( Fig. 24.19 ; Video 24.16 , Video 24.17 ); left-sided defects are much less common.

• Intraabdominal and extraabdominal dilated loops of bowel may be seen late in pregnancy ( Figs. 24.20 and 24.21 ).

• Complex gastroschisis if intraabdominal bowel dilation and/or liver or bladder herniation.

• Maternal serum alpha fetoprotein (AFP) is elevated with gastroschisis; with widespread use or prenatal screening, 95% of gastroschisis cases are diagnosed prenatally.

• Excluding omphalocele is important, as 40% of omphalocele cases are associated with aneuploidy; isolated gastroschisis is not typically associated with aneuploidy, making invasive prenatal testing unwarranted.

Differential Diagnosis

• Normal physiologic gut herniation ( Fig. 24.22 )

  • Occurs between 6th and 10th week of embryonic development.

  • Loop of intestines herniates, rotates around the superior mesenteric artery, and returns to abdominal cavity.

  • Gut herniation persisting beyond 12 weeks is likely gastroschisis.

• Ruptured omphalocele (umbilical cord located medially within the herniated intestinal mass, as opposed to intestines being lateral to the umbilical cord insertion as seen with gastroschisis).

• Limb–body wall complex (larger size abdominal mass, often adherent to placenta and/or with short umbilical cord).

• Pentalogy of Cantrell.

• Umbilical cord cysts.

• Urachal cysts.

• Loops of normal umbilical cord.

• Bladder or cloacal exstrophy.

See Fig. 24.23 for diagnostic algorithm for diagnosing suspected fetal abdominal wall defects.

Associated Anomalies

• Approximately 85% of gastroschisis cases occur in isolation.

• Other associated gastrointestinal malformations (complex gastroschisis), including bowel atresia, malrotation, volvulus with dilated loops of bowel, and perforation.

  • Vessels are constricted at level of abdominal defect.

  • Torsion of the mesentery leads to bowel ischemia and necrosis.

• Nongastrointestinal abnormalities, including arthrogryposis, cardiac defects, and genitourinary anomalies (e.g., bladder herniation), are infrequent (<5%).

• Oligohydramnios is common; however, polyhydramnios may accompany gastroschisis with concomitant bowel atresia or poor bowel function.

• Fetal growth restriction (FGR) is often present.

  • In one study of over 4000 live births with gastroschisis, the median birth weight was 2410 ± 300 g, delivering at a median 36 weeks’ gestational age.

  • FGR may be overestimated using standard Hadlock formulas for estimating fetal weight, as herniated abdominal contents result in an undersized abdominal circumference (AC).

  • Siemer (biparietal diameter [BPD] and femur length) or Shepard (modified AC and BPD) formulas may be more accurate for calculating estimated fetal weight in fetuses with gastroschisis.

Imaging