Twins may come into the world a little healthier if they share the womb with a sister rather than a brother, a new study suggests.

Israeli researchers found that among the 2,700 twin pregnancies they studied, the risk of preterm delivery was highest when both twins were male and lowest when both were female. In addition, the risks of certain complications, such as lower birthweight and newborn breathing problems, were higher among males and females who had shared the womb with a brother rather than a sister.

The findings, published in the journal Obstetrics & Gynecology, support the theory that the “male factor” raises the odds of pregnancy complications.

It’s known, for example, that boys are more likely than girls to be born prematurely, need a cesarean delivery or have newborn health problems. This latest study suggests that having a male twin may also raise the odds of complications.

However, the differences in risk are small, and the findings should not “panic” parents expecting male or mixed-sex twins, senior researcher Dr. Marek Glezerman, of Rabin Medical Center and Tel Aviv University, told Reuters Health in an email.

There are no immediate implications for managing twin pregnancies, Glezerman said. Instead, the findings are interesting because they suggest that the higher risk of problems with male pregnancies stems from a male “disadvantage,” rather than a female protective factor.

The study included 2,704 fraternal twin pregnancies; most were male/ female, while 16 percent were female/female and 14 percent male/male.

Overall, the risk of preterm birth was highest when both twins were male. Nine percent were born before the 31st week of pregnancy, compared with 7.5 percent of mixed-sex twins and 5.5 percent of female/ female twin pairs.

The “male factor” also appeared to have an effect on birth size. In general, the study found, baby boys had a lower birthweight and slower growth rate when they had shared the womb with a brother rather than a sister.

Girls showed an apparent disadvantage from having a male co-twin as well. Compared with baby girls who had shared the womb with a sister, they were more likely to have neurological complications or breathing problems from underdeveloped lungs.

Eleven percent of girls from male/female twin pairs developed respiratory distress syndrome, compared with 8 percent among female/ female pairs. And just over 1 percent developed convulsions, compared with none of the newborn girls from female/female pairs.

It’s possible, according to Glezerman’s team, that male sex hormones, such as testosterone, help explain at least some of the risks linked to having a male twin. These hormones have been found to limit lung development, for instance, and research in animals shows that female twins have higher testosterone levels when they share the womb with a brother.

Future studies, the researchers conclude, should look at whether having a male co-twin has any effects on long-term physical, mental or behavioral development.

SOURCE: Obstetrics & Gynecology, November 2009.