A shift from a male to a female majority in newborns with the increasing age of grand grand multiparous women

In this longitudinal study, we investigated the relationship of birth order and the age of mother and father to the gender of 1795 newborns (mean +/- SD 12.5 a 1.6 per mother) of 143 grand grand multiparas (i.e women who have had >10 deliveries). The frequency of boys was 52.2% in the group of Ist to 9th paras and 46.2% in the group of 10th to 20th paras (P = 0.022). Mothers aged greater than or equal to 35 years had 7.0% more female than male newborns (P = 0.024). The respective figure for fathers was 5.6% (P = 0.023). The interpregnancy interval evaluated for 96 mothers with 1091 deliveries had no correlation with the gender of the infants. In the stepwise logistic regression analysis, the age of the mothers remained the only significant independent factor for the shift from a male to a female majority in the newborns (P = 0.0389). The present data thus indicate that the age of the mother is the factor which explains why grand grand multiparous women deliver more girls than boys.