Effect of gravidity on maternal serum markers for Down's syndrome.

The effect of gravidity on maternal serum alpha-fetoprotein (AFP), unconjugated oestriol (uE3), and human chorionic gonadotrophin (hCG) levels was investigated in 27,592 women being screened for Down's syndrome. There was no difference in the median AFP level in primigravid and multigravid women, but the median hCG level in multigravid women was 5.9 per cent lower than in those tested in their first pregnancy (P < 0.0001) and the median uE3 level was 3.9 per cent lower (P < 0.0001). Among multigravid women, there was no material difference in hCG levels according to the number of previous pregnancies or livebirths, whereas uE3 levels declined steadily with increasing numbers. Both markers declined with advancing maternal age: for hCG this was an independent effect, but for uE3 it was due to the correlation between age and gravidity. Allowing for these effects will not greatly alter the Down's syndrome screening detection and false-positive rates.