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.

In the new study, birth weight was higher, on average, for boys than girls: 4.95 pounds, compared with 4.75. But boys in the boy-boy pairs had lower average birth weights than boys in the boy-girl pairs: 4.85 pounds versus 4.99. Boys in the boy-boy pairs also had lower growth rates than boys in boy-girl pairs.

Girls in the girl-girl pairs had fewer respiratory and neurological problems than those in the girl-boy pairs.

The researchers attribute the findings to what they call a "male offending factor," which means that the presence of a male fetus negatively affects the prospects of the co-twin in the womb, whether that co-twin is a sister or a brother.

However, the reasons for this aren't clear. The researchers speculate that one possibility is that male fetuses who share the womb with females could be more successful in competing for nutrients because females tend to grow more slowly. As a result, a boy in a boy-girl pair might end up weighing more than one in a boy-boy pair. Hormonal influences might help explain the differences, too.

None of this should worry expectant mothers, says another U.S. expert.

"Look at this study with a grain of salt and move on," said Dr. Abdulla Al-Khan, director of perinatal diagnostics and therapeutics in the maternal-fetal medicine division at Hackensack University Medical Center in New Jersey. The study's findings have not been noticeable, he said, at his hospital. They deliver more than 6,000 babies a year, including twins, he said, and women carrying twins are already followed closely to monitor the babies' progress.

Both Al-Khan and Ory also pointed out some of the study's limitations. It is retrospective, looking back, which they said makes it subject to bias. And, Al-Khan said, it's not clear whether the twins were conceived spontaneously or through fertility treatments, which might have affected the results.

SOURCES: Marek Glezerman, M.D., professor, Sackler Medical School, Tel Aviv University, and chairman, department of gynecology, Helen Schneider Hospital for Women, Rabin Medical Center, Tel Aviv, Israel; Steven Ory, M.D., fertility specialist, Margate, Fla.; Abdulla Al-Khan, M.D., director, perinatal diagnostics and therapeutics, division of maternal-fetal medicine, department of obstetrics and gynecology, Hackensack University Medical Center, Hackensack, N.J.; November 2009, Obstetrics & Gynecology