Industrial nations puzzle over decline in male birth ratio

It's a girl. And another girl. <br><br>Slightly more boys than girls are born in the United States, but the percentage of baby boys appears have gradually fallen during the last half of this century.

Monday, April 24th 2000, 12:00 am

By: News On 6

It's a girl. And another girl.

Slightly more boys than girls are born in the United States, but the percentage of baby boys appears have gradually fallen during the last half of this century. The loss of viewership on ESPN will probably be imperceptible: The actual decline has been small, representing less than 1 percent of births.

What does bother some scientists is that there is no clear reason this is happening, and why it is occurring in the most modernized countries of the world.

The theory that gets the most attention maintains that environmental pollution is toxic to some males while still in the womb. Certain pollutants are known to mimic the effects of female hormones when they leach into the body, and perhaps, the theory goes, some male fetuses aren't able to survive.

"I'm not certain there are environmental causes, but we have to think they are out there," said Devra Davis of the World Resources Institute, an environmental advocacy organization in Washington, D.C.

In 1998, Dr. Davis and her colleagues published a report in The Journal of the American Medical Association proposing that "reduced male proportion at birth be viewed as a sentinel health event that may be linked to environmental factors."

But scientists also have evidence for more benign influences on babies' gender, including the age of parents, family size, birth order, frequency of sexual intercourse, maternal nutrition and other subtle factors.

"It certainly doesn't seem like there is one thing that is out there," said Tom Sinks of the U.S. Centers for Disease Control and Prevention. And Dr. Sinks stresses that the deeper meaning of the trend isn't known.

"This is not a public health emergency," he said. "It's probably not meaningful in terms of the long-term survival of our species." However, he added, "If a particular chemical were adversely challenging male fetuses, we should be concerned."

But so far, data are fuzzy on the environmental question. For example, Dr. Sinks and his colleagues published research in 1998 in the journal Fertility and Sterility that found a decline in male births from 1969 to 1995. But the decline appears to be concentrated among white babies, which account for a majority of U.S. births. An environmental contaminant should affect all members of the population, Dr. Sinks said.

One of the most compelling studies to support the environmental explanation comes from tracking
births following a chemical plant explosion in 1976, which released a drifting plume of dioxin, a cancer-causing pollutant, over Seveso, Italy. Between April 1977 and December 1984, 74 children - 48 of whom turned out to be girls - were born to families who lived in the most exposed neighborhoods of the city. In addition, among the nine couples who had the highest levels of dioxin in their blood in 1976, none gave birth to a boy up to seven years after the explosion.

"You want to be very careful when you have a small study drawing large conclusions," Dr. Davis acknowledges. "That's an example of a very unusual exposure, but it makes you think there may be something going on with a smaller exposure over a large population."

Other researchers say the specter of an unknown environmental contamination may overshadow some of the simplest explanations. "I think that's raising a flag unnecessarily," said Mark Nicolich, a statistician with Exxon Biomedical Sciences in Annandale, N.J.

Dr. Nicolich is one of the researchers who has explored cultural, rather than environmental, culprits. While none of the suggestions has emerged as a clear answer, several reports in the past few months have highlighted certain possibilities.

For example, last month, Dr. Nicolich and his colleagues reported in Fertility and Sterility that the decline in male births could be partly explained by the increasing age of parents in the United States. Older parents, fathers especially, were more likely to have girls, he reported, and the association was particularly strong among white couples.

In fact, about 28 percent of the decline since 1964 among white births could be accounted for by changes in fathers' ages, and an additional 14 percent of the decline could be explained by taking into account the changes in mothers' ages.

Dr. Nicolich said he believes that other natural biological factors are probably the reason for the shrinking percentage of baby boys. And the true reasons may remain a mystery. "We'll never be able to explain it totally," he said.

Dr. Bob Biggar from the National Cancer Institute believes there is a simple answer. "I think it can be entirely explained on cultural grounds," he said.

What Dr. Biggar means, in particular, is family size. For whatever reason, big families have more boys, he has found. Last November, in the American Journal of Epidemiology, Dr. Biggar and his colleagues reported on a study of births in Denmark from 1960 to 1994. He chose Denmark because that country has detailed records on births and family size.

In general, his investigation revealed that "if you have had a male child, you are more likely to have another male child." If every household only had one child, his study found, 51.2 percent of births each year would be male. If families got larger, that percentage would go up.

Therefore, if family size gets smaller, the fraction of births that are male falls. And among most industrial nations, where the declines in male births are occurring, families have indeed gotten smaller, he said.

"It entirely explains the whole thing, in my opinion," Dr. Biggar said. "I don't think it's necessary to evoke toxins at all."

The famously detailed records of Scandinavian countries also recently enabled researchers from Finland to add perspective to the debate. Last October, in the journal Environmental Health Perspectives, the Finnish scientists looked back through 250 years of births in their country. From 1751 to 1920, they reported, the proportion of males increased. Then the proportion generally began to fall, although there were slight peaks in male births around the world wars. Unlike Dr. Nicolich's analysis of U.S. births, age of parents and other biological factors couldn't account for the decline.

But neither could pollution, the Finnish scientists said. The turning point in the trend, the 1920s, "precedes the period of industrialization or the introduction of pesticides or hormonal drugs, rendering a causal association unlikely."

Dr. Davis worries, though, that while researchers look for absolute proof, the trend will continue. Pollutants are already suspected of harming male wildlife, she said, so it would stand to reason that humans would not be protected.

"By the time we can prove that there's strong scientific grounds, it's too late," Dr. Davis said. "I think as a matter of public policy it makes more sense to take prudent precautions."

The CDC's Dr. Sinks said he hasn't dismissed any possibilities yet, although he's not convinced that something lurking in the environment is affecting males. "We clearly would want to know if there were some sort of chemical agent out there," he said. "It's worthy of a further look."