Parental aging synergistically decreases offspring sex ratio

Objective:  The aim of this study was to investigate the effect of parental age as a factor in the observed decline in the male to female birth ratio expressed as the offspring sex ratio (OSR). Study Design:  A prospective multicenter study was conducted from August 2005 to February 2007 at five community-based hospitals in Osaka, Japan. Pregnant women in the first trimester were recruited at their first prenatal care visit and followed until delivery. Multiple pregnancies and assisted conceptions were excluded. Periconceptional parental ages were recorded. Neonatal information was obtained at the time of delivery. Proportional distribution of categorical variables was studied using the χ2 or Fisher's exact tests (two-tailed). Results:  Data on 3,049 deliveries were available for review. OSR for the largest paternal and maternal subgroup (both, age 30–34) were male dominant (1.17 and 1.12, respectively). Paternal age ≥40 showed a smaller OSR (0.75 vs 1.17, P = 0.001). Advanced maternal age was associated with smaller OSR: age 35–39, 0.87 versus 1.12, P = 0.02; and age ≥40, 0.63 versus 1.12, P = 0.047. Synergistic effects of increasing paternal and maternal age on the OSR were observed. OSR for parental ages ≥40 were significantly smaller than ages 30–34 (0.52 vs 1.17, P = 0.029). Conclusions:  Increasing paternal ages synergistically decrease the male to female birth ratio.