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Women Born in Summer Have Fewer Children

But study's author can't say why

THURSDAY, April 29, 2004 (HealthDayNews) -- Women born in June, July and August have fewer children than women born in other months, a new study by Austrian researchers contends.

However, they can't pinpoint a reason for the finding. And an American expert in reproductive medicine said the study shouldn't worry women born in summer who are hoping for a houseful of children.

The study appears in the May issue of Human Reproduction.

The finding confirms a previous study of "pre-modern" women in Canada, as well as other studies, said Susanne Huber, a researcher at the University of Veterinary Medicine in Vienna.

"In contrast to the other studies all dealing with pre-modern women of historic populations, we are the first to investigate the association between birth month and reproductive performance in contemporary women," she said.

The researchers evaluated a representative sample of more than 3,000 Austrian women. They found that women born in July had 0.3 fewer children than women born in December, Huber said. Put another way, those women born in July had 13.4 percent fewer children than the 2.24 mean number of children borne by women overall, she said.

The average offspring count among women born in June, July or August was lower than those women born during any other month, Huber said.

The sample included women over the age of 45 and born after 1945 who were part of an Austrian database that has been ongoing since 1967.

Huber said that despite the potential influence of contraception and other realities of modern life, birth month effects on reproduction seem to remain the same in pre-modern and contemporary women.

An American expert called the study interesting, but said the findings might not hold up. "The data supports what they say, but these papers are very tricky," said Dr. Alan DeCherney, a professor of obstetrics and gynecology at the University of California, Los Angeles' David Geffen School of Medicine.

"If you look at enough variables, it will find something," added DeCherney, who is also editor of Fertility and Sterility. In the study, the researchers looked at 12 different variables, he noted.

Huber can't say exactly what's behind the birth-month effect she found. "We can only speculate about possible causes as this has not been investigated yet," Huber said. One possibility, she said, is that the lower fertility could be due to conditions experienced during fetal and neonatal life that affect early development before and immediately after birth.

"Our results are just of statistical relevance," Huber said. "That is, individual summer-born women may also have, for instance, 10 children. But on the average, women born in summer have fewer children."

While research like this attracts interest, DeCherney said, it may needlessly worry women born in summer hoping for children. "It's just going to alarm people born in June and July that their fertility is decreased," he said.

DeCherney's advice about the study's conclusion? "Disregard this."

More information

To learn more about women and reproductive health, visit the American College of Obstetricians and Gynecologists and the American Society for Reproductive Medicine.

SOURCES: Susanne Huber, Research Institute of Wildlife Ecology, University of Veterinary Medicine, Vienna; Alan DeCherney, M.D., professor, obstetrics and gynecology, University of California, Los Angeles' David Geffen School of Medicine; May 2004 Human Reproduction
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