The 11-14-week scan

Sonographically, the stomach is identified as a sonolucent cystic structure in the upper left quadrant of the abdomen. It is first visualized at 8–9 weeks and it is seen in all cases by 12–13 weeks^11,18,49. At 8–10 weeks, of gestation, all fetuses demonstrate herniation of the midgut that is visualized as a hyperechogenic mass in the base of the umbilical cord; retraction into the abdominal cavity occurs at 10–12 weeks and it is completed by 11 weeks and 5 days^11,67,68.

Exomphalos

This is a sporadic abnormality with a birth prevalence of about 1 in 4000. Prenatal diagnosis by ultrasound is based on the demonstration of the mid-line anterior abdominal wall defect, the herniated sac with its visceral contents and the umbilical cord insertion at the apex of the sac (Figure 12). Occasionally, there is an associated failure in the cephalic embryonic fold, resulting in the pentalogy of Cantrell (omphalocele, anterior diaphragmatic hernia, sternal cleft, ectopia cordis and cardiac defects) or failure of the caudal fold, in which case the omphalocele may be associated with exstrophy of the bladder or cloaca, imperforate anus, colonic atresia and sacral vertebral defects. Fetal exomphalos is associated with chromosomal defects, usually trisomy 18, in about 30% of cases at mid-gestation and in 15% of neonates.

Schmidt and Kubli described a case of exomphalos at 13 weeks as an echogenic tumor at the umbilicus; the fetus was subsequently found to have trisomy 18¹⁹. Brown et al. reported the diagnosis of exomphalos containing liver at 10 weeks, but retrospective examinations of the sonograms obtained at 6–9 weeks did not reveal any abnormality; the diagnosis was confirmed after delivery⁶⁹. Similarly, Pagliano et al. reported the diagnosis of exomphalos containing liver and bowel in a 10-week fetus; the pregnancy was terminated and the diagnosis was confirmed⁷⁰. Heydanus et al. reported the diagnosis of exomphalos in three fetuses at 12–14 weeks; in one there was an associated ectopia cordis and hydrops and the pregnancy was terminated, in the second there was an associated two-vessel cord and intrauterine death occurred and, in the third with isolated exomphalos, there was an infant death⁷¹.

van Zalen-Sprock et al. reported the findings of 14 cases with exomphalos diagnosed at 11–14 weeks of gestation⁶⁸. In eight cases, there was increased nuchal translucency thickness (3.5–10 mm) and seven of these had chromosomal abnormalities, mainly trisomy 18. The contents of the exomphalos were bowel only in the chromosomally abnormal group and liver as well as bowel in those with a normal karyotype. In the chromosomally normal group, there were four with other defects, such as tetralogy of Fallot and Meckel–Gruber syndrome; only three infants were liveborn.

Screening studies

An ultrasound screening study of 622 high-risk pregnancies at 10–13 weeks correctly diagnosed the two cases of exomphalos²⁵. In two other screening studies of low-risk patients, involving 1632 pregnancies at 12–14 weeks³⁵ and 1473 pregnancies at 10–14 weeks²⁷, respectively, there were four cases of exomphalos (two in each) and they were all diagnosed in the first-trimester scan.

In a screening study for chromosomal abnormalities by assessment of fetal nuchal translucency thickness at 10–14 weeks of gestation, there were 15 726 pregnancies with a minimum gestation of 11 weeks and 4 days and, in this group, there were 18 cases of exomphalos⁷². In seven cases, the karyotype was normal, in nine there was trisomy 18, in one trisomy 13 and in one triploidy. Furthermore, in the total group, the prevalence of exomphalos in fetuses with trisomy 18 was 23%, in those with trisomy 13 it was 9%, in those with triploidy it was 13% and in those with no evidence of these chromosomal defects it was 0.045%. This study demonstrated that both the prevalence of exomphalos and the associated risk for chromosomal defects increase with maternal age and decrease with gestational age⁷².

Gastroschisis

This is a sporadic defect with a birth prevalence of about 1 in 4000. Evisceration of the intestine occurs through a small abdominal wall defect located just lateral and usually to the right of an intact umbilical cord. Prenatal diagnosis by ultrasound is based on the demonstration of the normally situated umbilicus and the herniated loops of intestine, which are free-floating. Associated chromosomal abnormalities are rare.

Surprisingly, although the incidence of gastroschisis in ultrasound studies during the second trimester of pregnancy is similar to that of exomphalos, there is a sparsity of reports on first-trimester diagnosis. Kushnir et al. reported a 13-week fetus with a free-floating cauliflower-shaped mass protruding through the fetal abdomen and to the right of a normally inserted umbilical cord; the diagnosis was confirmed after delivery at term⁷³. Similarly, Guzman reported a 12-week fetus with gastroschisis; there was spontaneous rupture of membranes and intrauterine death at 22 weeks⁷⁴.

Screening study