Excess males in preterm birth: interactions with gestational age, race, and multiple birth

To confirm an excess of males among preterm births and study its interaction with other risk factors to better understand its potential etiologic significance.

Fetal gender was analyzed in 1,781,960 white and 103,329 black singleton births, and in 37,429 white twin births using vital statistics data from the six New England states for 1977-1988.

A 7.2% excess of males was found among white singleton preterm births. There was only a 2.8% excess among comparable blacks, a highly significant difference between the races (P < .001). The effect was roughly constant for 20-37 weeks' gestation. Being married increased the effect for white, but not black, women, and was also found among white fetal deaths. Among white twins, male excess in preterm births occurred only in 20-33 week's gestations.

Male fetal gender is associated with singleton preterm birth, an effect most evident in white women, particularly if married. Among preterm white twins, there is also a male excess, limited to gestations under 34 weeks. The excess of males in selected groups suggests the existence of a mechanism of preterm birth influenced by fetal gender. Preterm births in blacks and in twin gestations greater than 33 weeks may be more often due to alternative mechanisms that are independent of fetal gender.