Sex of Twins May Affect Outcomes

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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.

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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.

More information

The American Academy of Pediatrics has more about twins.

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