EFFECT OF GRAVIDITY OF MATERNAL SERUM MARKERS FOR DOWNS-SYNDROME

The effect of gravidity on maternal serum a-fetoprotein (AFP), unconju gated oestriol (uE(3)), 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 mul tigravid 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 uE(3) level was 3.9 per cent lower (P<0.0001). Among multigravid women, there was no material difference in hCG levels acco rding to the number of previous pregnancies or livebirths, whereas uE( 3) levels declined steadily with increasing numbers. Both markers decl ined with advancing maternal age: for hCG this was an independent effe ct, but for uE(3) it was due to the correlation between age and gravid ity. Allowing for these effects will not greatly alter the Down's synd rome screening detection and false-positive rates.