Embryological development was investigated by ultrasonography in a longitudinal study of 18 normal pregnancies. The appearance of midgut herniation was studied and if present, its circumference was valued. In addition, patients with an elevated risk for anomalies were scanned between 10 and 16 weeks of gestation. In 17 fetuses, the diagnosis of omphalocele was made between 11 and 16 weeks. The ratio between the circumference of the omphalocele and abdomen was calculated and the contents of the omphalocele were evaluated. Eight of the fetuses had an abnormal karyotype and a mean omphalocele/abdomen ratio of 0.38 (SD 0.191), whereas the mean ratio of omphalocele and abdomen in the eight euploid fetuses was 0.88 (SD 0.291). Regarding the contents of the omphalocele, mainly bowel loops were eviscerated in all fetuses with an abnormal karyotype. The contents of the omphalocele sac in the euploid cases contained liver in all eight fetuses. In 16 out of 17 anomalous fetuses (91 per cent), associated findings were detected. Nuchal translucency was the most frequently visualized concurrent finding and its occurrence was associated with an abnormal chromosome pattern. A small sac size is a strong indication for aneuploidy, especially in combination with nuchal translucency. Prior to 12 weeks of gestation, the diagnosis of omphalocele can be made in cases of a large omphalocele.