Battles between the sexes have been waged endlessly, but a new study gives 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 recently concluded that outcomes for twin pregnancy are enhanced when at least one of the twins is a girl. The 2009 study evaluated more than 2,500 twin pregnancies and constrasted the outcomes for girl-girl, boy-girl, and boy-boy twins.
The researchers found that the incidence of preterm deliveries was 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.
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. They also theorized that males may compete for nutritional resources more favorably for against a female fetus, which grows more slowly, increasing fetal weight gain for the male.
However, experts acknowledge many limitations associated with the study. 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 agreed that the study results should not change the way that twin pregnancies are treated.