Impact of holoprosencephaly, exomphalos, megacystis and increased nuchal translucency on first-trimester screening for chromosomal abnormalities.

OBJECTIVES To examine the prevalence of alobar holoprosencephaly, exomphalos, megacystis and nuchal translucency thickness (NT) ≥ 3.5 mm, the incidence and types of chromosomal abnormalities associated with these conditions and their overall impact on the rate of invasive testing and performance of screening at 11-14 weeks. METHODS This was a prospective screening study for trisomies 21, 18 and 13 by the first-trimester combined test at three maternity units in England. RESULTS In the study population of 108 982 singleton pregnancies, 870 (0.8%) had abnormal karyotype, including 654 (75.2%) with trisomies 21, 18 or 13 and 216 (24.8%) with other chromosomal abnormalities. The prevalence of alobar holoprosencephaly, exomphalos, megacystis and NT ≥ 3.5 mm was 1 in 2945, 1 in 419, 1 in 1345 and 1 in 119, respectively. Chromosomal abnormalities were observed in 78.4% of cases of holoprosencephaly, 40.8% of exomphalos, 18.5% of megacystis and 48.5% of those with NT ≥ 3.5 mm. The most common chromosomal abnormality associated with holoprosencephaly was trisomy 13, with exomphalos and megacystis was trisomy 18 and with increased NT was trisomy 21. Fetal karyotyping of cases with major fetal defects or increased NT would potentially detect 57% of all chromosomal abnormalities at an invasive testing rate of 1.1%. CONCLUSION Major fetal defects and increased NT at 11-13 weeks' gestation are associated with a high risk of chromosomal abnormalities and merit invasive fetal testing. Copyright © 2016 ISUOG. Published by John Wiley & Sons Ltd.