In Science News, J. Raloff writes that boys’ birth rates are falling and scientists aren’t sure why. During fetal development, "the male sex is clearly the more fragile one," says Bruce B. Allan, an obstetrician-gynecologist in Alberta, Canada. While some 125 males are conceived for every 100 females, only about 106 boys are actually born for every 100 girls. Stillbirths and miscarriages disproportionately affect boys.

Devra Lee Davis of the World Resources Institute (WRI) in Washington, D.C., and her colleagues have analyzed recent surveys by Allan and others who have looked at trends in sex ratios in several countries. They find a broad pattern of decling male births and increasing reproductive defects.

"There are compelling biological reasons [for this," says Davis. She suspects this can be traced to the disruption of normal male fetal development by environmental agents.Last year, Allan?s team studied sex ratios in Canada from 1930 to 1990. The proportion of males rose for 2 decades — a trend that they think was caused by the nation?s improving health care.

The ratio then held steady until 1970, when "we saw a drop in every region," Allan says. The most significant drop, which was twice the drop in the rest of Canada, occurred in the Atlantic provinces. The sex ratio at birth "is a parameter that really shouldn’t change" in a healthy, well-cared-for population, Allan says. His team decided to analyze U.S. births over the same period. "And though more subtle, the male sex ratio was dropping there, too," he found, by about 1 boy per 1,000 births.

Over the past 2 years, Dutch and Danish researchers have reported similar drops in the male birthrate in their countries. Just last month, APMIS, a journal published in Denmark, reported declining male births in Sweden, Germany, Norway, and Finland.Evidence of a decline over such a broad geographic area, according to Michelle B. Gottlieb, of the WRI, "suggests that avoidable, environmental, factors may be playing a role." Though certain diseases, older parents, and fertility-stimulating drugs have all been linked to an increasing proportion of female births, Gottlieb?s team found that these factors could explain only a small part of the trend. Several recent studies point to hormone-like pollutants.

A 1996 study reported the sex of children born to couples who had been exposed to large amounts of dioxin during a July 1976 industrial accident near Seveso, Italy. In the first 8 years after the accident, 12 daughters — and no sons — were born to the nine couples who had more than 100 parts per trillion (ppt) of dioxin in blood samples taken at the time of the accident.

Among the four couples whose dioxin concentrations were below 100 ppt, the male-female ratio approached normal, says Larry L. Needham, of the Centers for Disease Control and Prevention, who helped analyze the blood samples. He?s now examining another 1,000 samples from other Seveso victims.Allan?s study "focuses attention on a trend that people might not have noticed by viewing individual studies," says Shanna Swan, of the the California Department of Health Services in Berkeley. "The consistency of the data is quite compelling — and lends biological plausibility that [this trend] might be due to environmental chemicals."

Boston University epidemiologist Richard Clapp believes that by looking for "hot spots" where sex ratios are especially skewed, "we might now get closer to finding the causative agents."

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