The influence of fetal sex in screening for Down syndrome in the second trimester using AFP and free beta-hCG.

In a study of 30 802 unaffected pregnancies and 90 cases of Down syndrome I have analysed second trimester maternal serum alpha-fetoprotein (AFP) and free beta-human chorionic gonadotrophin (beta-hCG) and grouped the data by fetal sex. In pregnancies with an unaffected female fetus maternal serum AFP was significantly lower (3%) than in the presence of a male fetus, whilst free beta-hCG was significantly raised (7%) in the presence of a female fetus. These differences led to a significantly higher false positive rate in women carrying a female fetus (5.78% versus 4.64%). In pregnancies affected by Down syndrome the median AFP in the presence of a female fetus was not significantly different to that with a male fetus (0. 72 versus 0.73); similarly for free beta-hCG the difference (2.37 versus 2.48) was not statistically significant. Although the detection rate amongst male fetuses was slightly higher than in females (74% versus 66%), this was not statistically significant. Although fetal gender does appear to have a significant effect on maternal serum marker levels, leading to a higher false positive rate in females, in cases of Down syndrome there is no evidence that such marker differences are significant and consequently little evidence for any fetal gender bias in detection rates.