He Ain't Wispy, He's My Brother

In twin sets, girl makes boy the heavyweight, study shows

TUESDAY, Aug. 21, 2001 (HealthDayNews) -- A new Belgian study may set the notion of sibling rivalry on its ear: In a set of girl/boy twins, it seems, the girl controls the boy's birth weight, making him heavier.

In a study of all twins born in the Belgian province of East Flanders, researchers found that a female twin lengthens the development time in the womb. The result for the brother is just a few extra days in the womb and just a couple of ounces of weight bounce, but enough to make a difference.

Studies have shown that females develop in the womb longer, but "worldwide birth weight of boys is higher than that of girls," author Ruth Loos, a researcher with the Center for Human Genetics in Leuven, Belgium, says in an e-mail interview. "It is generally known that length of gestation is the most important determinant of birth weight. . . and since we are involved in twin studies we wondered what would occur when a boy and a girl share the womb."

Loos and her colleagues compared the birth weight and length of pregnancy of 1,929 sets of fraternal twins, in what she calls the world's most comprehensive twin registry.

"Our results showed that. . . female-female twins had a longer gestation (0.4 weeks) than male-male twins and that, at birth, male-male twins weigh more than female-female twins," Loos says.

"More interesting is the case of the opposite-sex twins: gestation is as long as that of female-female twins, so the presence of a girl is sufficient to prolong gestation for the brother," she adds.

In addition, she notes, "the boy of an opposite-sex pair [of twins] benefits from the longer gestation, because he weighs more -- 78 grams (2.73 ounces) -- than boys of male-male pairs."

Fraternal twins, who come from two eggs, are twice as common as identical twins, who develop from one egg, according to the U.S. Centers for Disease Control and Prevention. Unlike identical twins, fraternal twins often have a different gender and other traits.

Hormones may explain the higher birth weight and the length of gestation, Loos suggests. "The birth weight of boys is thought to be generated by androgen action," she says. Androgens are hormones produced in the ovaries and adrenal glands that act on the musculoskeletal, nervous, liver and vascular tissues.

"Androgen not only increased their birth weight, but also influences the timing of the fetal growth," she adds. "Boys grow faster in the first part of gestation, whereas girls show a partial 'catch-up' later in gestation."

"It's hard to speculate why girls prolong gestation," Loos continues. "Fetal hormones interact with the maternal hormones to initiate the delivery. Maybe in the case of female fetuses, the signal to deliver is postponed and delivery is delayed. However, more research into endocrinological factors of the onset of delivery is needed."

The findings were published in the Aug. 18 issue of The Lancet.

"What Loos is showing is that there is something about this prenatal environment that is affecting the pregnancy, and that's significant," says twin expert Nancy L. Segal, a professor of psychology at the California State University, Fullerton, and author of the book Entwined Lives.

"The suggestion, therefore, is that there could be something about the prenatal environment that affects behavior later in life. But the research on that is very mixed," she says.

"What's unique here is that these fetuses are exposed to cross-gender hormones, and there have been some studies on this, but what we've gotten is mostly a very mixed picture," Segal says. "If you look at animals like gerbils, rats or mice, mostly what you'll find is a masculinizing effect on the females of a multi-sex litter."

Some studies have found some small differences in hearing and teeth structure, but not much else. "They found no difference in the average age of first menstruation in opposite-sex twins compared to same-sex twins, and girls from opposite-sex pairs showed better spatial visualization than other girls," Segal says.

Whether it's being in the womb with their brothers or growing up with them, differences have been hard to tease out, Segal insists.

"They share both the prenatal and the postnatal environment," she says. "So how do you know which is affecting what?"

What To Do

For more information on twins, see the Twins Foundation or the National Organization of Mothers of Twins.

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