Battles between the sexes have been waged endlessly, but a 2009 study gave a definite edge to females. Researchers at the Helen Schneider Hospital for Women and the Sackler School of Medicine at Tel Aviv University in Israel concluded that outcomes for twin pregnancy are enhanced when at least one of the twins is a girl. The study evaluated more than 2,500 twin pregnancies and contrasted the outcomes for girl-girl, boy-girl, and boy-boy twins.

Girl vs. Boy Twins

The researchers found that the incidences of preterm deliveries were higher in boy-boy twins, and the babies had lower average birth weights and lower growth rates when both twins were male. Meanwhile, girl twins had fewer respiratory and neurological problems. Interestingly, the results showed that it only took one girl to improve the outcome for a boy; across the board, boys with a twin sister fared better than sets of boy twins.

  • The boy twins had a lower mean birth weight and a lower growth rate.
  • The girl twins had fewer problems with their brains and lungs.
  • The risk of an early delivery (before 31 weeks) was highest in the male–male twins.

Researchers cited a “male offending factor,” explaining that interfetal transport of hormonal substances from a male fetus has a negative effect on the other twin.

Research Considerations

Other studies of premature infants recognize that female babies have an advantage. For example, in a 2016 study of singleton infants, the boys had significantly higher rates of some complications, even though they generally weigh more than girls at birth. Higher incidences of disabilities were also associated with males.

Researchers theorized that male twins may compete for nutritional resources more favorably against a female fetus, which grows more slowly, increasing fetal weight gain for the male.

However, many limitations associated with this study of twins have been acknowledged. For one, it only studied twins with two separate placentas, excluding a portion of monozygotic twins.

Also, it does not distinguish between multiples conceived spontaneously or with fertility assistance, which could affect the pregnancy outcome. More than two-thirds of the twin sets in the study were male-female pairs, with about 15% being same-sex twin sets. Finally, the retrospective nature of the study may make it subject to bias. Most doctors would agree that this study’s results should not change the way that twin pregnancies are treated.