The gentlebirth.org website is provided courtesy of
Ronnie Falcao, LM MS, a homebirth midwife in Mountain View, CA

Girl Twin May Determine Pregnancy Length

Friday August 17, 2001 10:37 AM ET

By Suzanne Rostler

NEW YORK (Reuters Health) - In different-sex twins, the female calls the shots when it comes to how long the pair is going to remain ensconced in the womb, a new report suggests.

Belgian researchers found that girl-boy twins tend to stay in the womb a little longer than do same-sex male twins. And this extra time allowed boys to pack on slightly more weight before birth.

The findings may help shed light on how labor is initiated by male and female fetuses and explain why certain babies are born with a higher risk of developing certain disorders later in life, Dr. Ruth Loos,  the study's lead author, told Reuters Health.

According to some scientists, fetuses that grow quickly are at risk for inadequate nutrition during pregnancy, which could predispose them to certain metabolic disorders as adults, explained Loos, from the Centre for Human Genetics in Leuven, Belgium.

In general, male fetuses tend to grow faster at an earlier stage while female fetuses go through growth spurts later.

"In twin pregnancies, the risk for under-nutrition is especially high in the last trimester. Therefore, twin girls, who grow fast in late pregnancy, may have an increased risk of developing metabolic diseases,''  Loos said.

In their study, the researchers compared the birth weights of more than 1,900 sets of twins. The findings, reported in a letter in the August 18th issue of The Lancet, reveal that gestational length was similar for same-sex female twins and for male-female twins--about 37 weeks. Same-sex male twins, however, had an average gestational period of just over 36 weeks.

What's more, the slightly longer stay in the womb for males from different-sex twins resulted in a higher average birth weight (about 78 grams, or 2.75 ounces), compared with that of boys from same-sex pairs.

"These data show that in unlike-sex pairs it is the girl that prolongs gestation for her brother, resulting in a higher birth weight than that of same-sex boys,'' Loos and colleagues conclude.

It is unclear why girls determine the length of gestation, but Loos suspects that the mechanism may relate to female hormones.

"Maybe in the case of female fetuses the signal to deliver is postponed and labor delayed,'' she said. "However, more research into (hormonal) factors of the onset of delivery is needed.''

SOURCE: The Lancet 2001;358:560-561.