LAUSR

BACKGROUND Down syndrome (DS) ranks as one of the most common birth defects in the United States. The relaxed filter hypothesis (RFH) asserts that mothers with no previous children but nearing reproductive senescence reduce fetal selectivity against DS pregnancies in the first trimester given the high probability of having no offspring at all if the DS pregnancy is not carried to term. We test the parity prediction of RFH-specifically, that pregnancies to older mothers with no previous live births show an elevated prevalence of DS above values expected from the separate contributions of older maternal age and primiparity. METHODS We retrieved maternal age and parity information on 2,748 DS cases (live, stillborn, or terminated due to anomaly) from the Paris Birth Defects Registry, 1983-2015. We used two waves of the Enquête Fertility Survey (n = 5,460) and counts of stillbirths to calculate total prevalence of DS by maternal age and parity. RESULTS Primiparous mothers 35+ years show the greatest prevalence of any parity group (9.78 cases per 1,000 deliveries). Tests for additive interaction of primiparity and older maternal age reject the null (relative excess risk due to interaction = 1.01; 95% confidence interval: 0.33, 1.69; null value on additive scale is 0). CONCLUSIONS Relaxed selection may account for 10% of the increase in the prevalence, among older primiparous mothers, of DS detected during or after the 11th week. Future work may hold implications for understanding, among older mothers, the strength of this maternal screen against other defects.