The 11-13-week scan: diagnosis and outcome of holoprosencephaly, exomphalos and megacystis.

OBJECTIVE To determine the prevalence and outcome of fetuses with holoprosencephaly, exomphalos and megacystis diagnosed at 11-13 weeks of gestation. METHODS As part of a prospective screening study for trisomy 21 in singleton pregnancies at 11 + 0 to 13 + 6 weeks' gestation, transabdominal ultrasound examination was performed to diagnose holoprosencephaly, exomphalos and megacystis. Fetal karyotype and pregnancy outcome in fetuses with these defects were examined. RESULTS Screening was carried out in 57 119 pregnancies. The prevalence of holoprosencephaly, exomphalos and megacystis was 1 : 1298, 1 : 381 and 1 : 1632, respectively. Chromosomal abnormalities, mainly trisomies 18 and 13, were found in 65.9% of fetuses with holoprosencephaly, in 55.3% with exomphalos and in 31.4% with megacystis. There was spontaneous resolution of the defect by 20 weeks in 92.5% of euploid fetuses with exomphalos containing only bowel and in 90% of the euploid fetuses with megacystis and bladder length of < or = 15 mm. CONCLUSIONS A high proportion of fetuses with holoprosencephaly, exomphalos and megacystis diagnosed at 11-13 weeks of gestation are aneuploid, but in the majority of cases exomphalos and megacystis represent temporary abnormalities that resolve spontaneously.