Fetal exomphalos and chromosomal defects: relationship to maternal age and gestation.

In an ultrasound screening study involving 15,726 viable, singleton pregnancies at 11-14 weeks of gestation, exomphalos was diagnosed in 0.11% of the cases and, in those with exomphalos, the frequency of trisomy 18, trisomy 13 or triploidy was 61%. The corresponding frequencies of exomphalos of fetuses with these chromosomal defects were 22.5%, 9.1% and 12.5%, respectively. The median maternal age of the screened population was 33 (range 15-48) years, which is higher than in all pregnancies in England and Wales. Expected prevalences of trisomy 18, trisomy 13 and triploidy in the total population were derived on the basis of the age distribution of all deliveries in England and Wales and maternal and gestational age-specific risks for these chromosomal defects. From these numbers and the observed frequencies of exomphalos in association with the various chromosomal defects, it was estimated that the prevalence of exomphalos in a population with the maternal age distribution of all deliveries in England and Wales was 7.4 per 10,000 at 12 weeks of gestation, and this decreased to 3.5 at 20 weeks and 2.9 in live births. The estimated frequency of chromosomal defects in fetuses with exomphalos decreased from 39.4% at 12 weeks of gestation to 27.5% at 20 weeks and 14.4% in live births. The prevalence of chromosomal defects in 153 fetuses with exomphalos referred to our center at 16-26 (median 20) weeks of gestation was not significantly different from that predicted in an unselected population. However, the reported frequency of chromosomal defects in a total of 299 neonates with exomphalos (9.3%) was significantly lower than expected in an unselected population. This study demonstrates that the prevalence of a fetal abnormality and the frequency of associated chromosomal defects depends on the maternal age and gestational age distributions of the populations examined.