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Inhibin‐A concentrations between 14 and 22 weeks of gestation

Inhibin-A is a useful screening marker in prenatal screening for Down syndrome, particularly in the second trimester of pregnancy. Inhibin-A concentrations decline from about 14 until about 16–17 weeks of pregnancy, and then slowly increase. A quadratic regression model (using log inhibin-A concentrations) has been used to describe the change in concentrations (Watt et al., 1998, 1999) but recently, with data becoming increasingly available from screening programmes, it has emerged that the quadratic model fits poorly before 16 and after about 21 weeks of gestation. These observations prompted us to try to find a better mathematical function that would fit the change in inhibin-A concentrations between 14 and 22 weeks of pregnancy. We used data from 104 615 non-smoking women who had singleton pregnancies without Down syndrome from our routine screening service. Inhibin-A concentrations were measured using an automated ELISA assay (Diagnostic Systems Laboratory, Houston). Figure1 shows the median concentration of inhibin-A in 2-day periods from 14 weeks 0 days to 22 weeks 6 days, together with 95% confidence intervals for each period. The top part of the figure (labelled a) uses a log-quadratic regression, and the bottom part (labelled b) uses a log-cubic regression line. Each regression was weighted by the number of women in each 2-day period. The figure gives the number of pregnancies tested in each completed week of pregnancy. The log-cubic regression was a significantly better fit to the data than the log-quadratic regression (p < 0.001). The equation for the log-cubic regression is: