The pattern of maternal serum inhibin-A concentrations in the second trimester of pregnancy

Maternal serum inhibin-A concentration is a useful marker in prenatal screening for Down syndrome in the second trimester. We measured inhibin-A concentrations in 4304 pregnancies without Down syndrome between 14 and 22 weeks of pregnancy to determine the median values according to gestational age. There was a U-shaped pattern of inhibin-A concentration against gestational age with a minimum concentration at 17 weeks and 1 day (120 days). We suggest that screening centres use a quadratic equation when estimating their normal median inhibin-A level, constraining the shape of the curve and fixing the lower point of the curve to occur at 120 days, but allowing its position in relation to the vertical inhibin-A axis to be set according to the local data. This approach will systematically allow for the changing inhibin-A concentration without introducing the instability of deriving a fresh quadratic equation for each screening centre and each time a centre's medians are revised.