Sex of Twins May Affect Outcomes

Oct. 22 -- WEDNESDAY, Oct. 21 (HealthDay News) -- The outcomes of a twin pregnancy -- including the infants' size and delivery date -- might be affected by whether a woman is carrying boys or girls.

Sharing the womb with a female produces better results, in terms of birth weight and other factors, than sharing it with a male, whether the other twin is male or female, according to researchers from the Helen Schneider Hospital for Women and the Sackler School of Medicine at Tel Aviv University in Israel.

"A male fetus affects his co-twin negatively, probably due to the interfetal transport of substances, mainly hormones," said Dr. Marek Glezerman, a university professor and chairman of the hospital's obstetrics and gynecology department and a co-author of the study.

But a U.S.-based expert who reviewed the study calls it interesting but no cause to change the way pregnant women are followed during their pregnancies, regardless of whether they are carrying two girls, two boys or one of each.

"I don't think it translates to changes in clinical practice," said Dr. Steven Ory, a past president of the American Society for Reproductive Medicine and a fertility specialist in Margate, Fla.

The study is published in the November issue of Obstetrics & Gynecology.

The researchers evaluated 2,704 twin pregnancies, looking only at twins within separate placentas born from 1995 through 2006. Previous studies had not differentiated between twins with common or separate placentas, Glezerman said, and this could have blurred the effect of the fetus's sex on the outcome of the pregnancies.

About 16 percent of the pregnancies involved female-female twins, 70 percent were male-female and 14 percent were male-male.

Preterm deliveries were most common among women carrying male-male twins, the study found. Of the male-male twins, 9.2 percent were delivered at less than 31 weeks, compared with 7.5 percent of the male-female twins and 5.5 percent of the female-female twins. Earlier studies have found a higher risk for premature delivery for a male fetus compared with a female.