Alternative methods of maternal weight adjustment in maternal serum screening for Down syndrome and neural tube...

Serum markers used in screening for Down syndrome and neural tube defects are often adjusted to take account of the effect of maternal weight on the marker levels. The standard adjustment procedure is based on a linear relationship between the marker concentration, expressed as the log of the multiple of the median (MOM), and maternal weight on a linear scale. It has been proposed that maternal weight adjustment may be better performed using a linear relationship between marker concentration expressed in MOM and the reciprocal of maternal weight. In a dataset of 8905 singleton pregnancies in white women without Down syndrome or neural tube defects we compared the two methods of weight adjustment and found that both were satisfactory and neither had an obvious advantage over the other. In the analysis it was noticed that hCG levels in very heavy women (> 120 kg) were higher than expected from the decreasing linear trend with maternal weight--a result that was statistically highly significant (p < 0.01) but for which we have no explanation. In screening it will have virtually no effect because the finding was restricted to only the 0.3 per cent of the heaviest women.