PREVALENCE OF FETAL FACIAL CLEFT AT DIFFERENT STAGES OF PREGNANCY

During a 7-year period (1988-94), we diagnosed 102 fetuses with trisom y 18, and 54 with trisomy 13; in 6.9% of the trisomy 18 and in 40.7% o f the trisomy 13 fetuses, there was a facial cleft. On the basis of (1 ) these frequencies of facial cleft in trisomic fetuses; (2) the repor ted prevalence of facial cleft in mie-trimester fetuses; and (3) estim ates of the prevalence of trisomies 18 and 13 at 20 weeks of gestation in a population with the maternal age distribution of all deliveries in England and Wales, it was calculated that 6.55 of fetuses with a fa cial cleft would have trisomy 18 or 13. This estimated frequency of tr isomies was significantly lower than 26% observed in 111 fetuses with a facial cleft that were referred to our unit for fetal karyotyping. T hese findings suggest that the patients with a facial cleft examined i n a referral center are preselected in favour of those with multiple a bnormalities, and therefore a higher frequency of associated chromosom al defects. In the future, with improving quality of ultrasound equipm ent and standards of scanning, it is likely that more cases of isolate d facial cleft will be identified and, consequently, the observed freq uency of chromosomal defects should decrease.