Refinements in managing maternal weight adjustment for interpreting prenatal screening results.

This study examines the relationship between maternal weight and serum levels of alpha-fetoprotein, unconjugated oestriol, and human chorionic gonadotropin in a population of 47,585 women being provided with prenatal screening for Down's syndrome and open neural tube defects. The study population contains sufficient numbers of women at the extremes of weight to allow the determination that a reciprocal-linear equation more accurately describes the weight relationship for two of the three analytes than the currently used log-linear equations. The reciprocal-linear equations, while more appropriate, provide only a minimal advantage over the log-linear equations. A more important finding is that published weight equations may not be optimal for some screening programmes, due to differences in the mean weight of the populations being tested. Screening programmes are encouraged to calculate their own weight correction formulae, based on data from their own population, and to monitor the mean maternal weight to detect when modifications in the weight correction formulae might be indicated.