Triploidy identified through second‐trimester serum screening

Objectives To describe the maternal serum marker patterns of triploid pregnancies and estimate the second-trimester prevalence of triploidy. Methods Forty-two cases of triploidy were identified in six serum screening programmes, five in the United Kingdom, one in Canada. This study describes the serum marker patterns, serum screening results for Down syndrome, trisomy 18 and open neural tube defects, and maternal age of these triploidy cases. The risk cut-off levels were ≥ 1 in 250 for Down syndrome, ≥2.5 MoMs alpha-fetoprotein for open neural tube defects and ≥1:100 for trisomy 18 screening. The estimated second-trimester prevalence of triploidy was based on 22 triploidy cases ascertained in 599 934 pregnancies from three routine screening programmes, which attempted complete ascertainment of aneuploidy cases. Results The observed second-trimester rate of triploidy was 0.37 per 10 000 fetuses. Two different serum marker patterns were seen in triploid pregnancies, distinguished from each other by typically very high or very low levels of total hCG or free β-hCG. The median maternal ages were respectively 33 years for triploidy with human chorionic gonadotrophin levels < 1.0 MoM, and 26 years for those with hCG levels ≥1.0 MoM. Fifty-seven percent of the pregnancies with a triploid fetus had a risk estimate ≥1:100 for trisomy 18 alone, 10% had an alpha-fetoprotein ≥2.5 MoM, 5% were screen positive for Down syndrome alone, and 19% had an increased risk or positive results for more than one anomaly. Conclusion The simultaneous use of maternal serum tests designed to screen prenatally for Down syndrome, neural tube defects, and an increased risk of trisomy 18 resulted in a screen-positive result for 90% of pregnancies with triploidy. Copyright © 2005 John Wiley & Sons, Ltd.