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 orderand the age of mother and father to the gender of 1795 newborns (mean +/-SD 12.5 +/- 1.6 per mother) of 143 grand grand multiparous (i.e women whohave had >10 deliveries). The frequency of boys was 52.2% in the groupof 1st to 9th paras and 46.2% in the group of 10th to 20th paras (P =0.022). Mothers aged > or =35 years had 7.0% more female than malenewborns (P = 0.024). The respective figure for fathers was 5.6% (P =0.023). The interpregnancy interval evaluated for 96 mothers with 1091deliveries had no correlation with the gender of the infants. In thestepwise logistic regression analysis, the age of the mothers remained theonly significant independent factor for the shift from a male to a femalemajority in the newborns (P = 0.0389). The present data thus indicate thatthe age of the mother is the factor which explains why grand grandmultiparous women deliver more girls than boys.