A new study has renewed the debate among public health experts over figures that seem to show a gradual but persistent decrease in the ratio between the birth of baby boys and baby girls.
The authors of the study, published in this week's edition of the online journal Environmental Health Perspectives, point out that the proportion of baby boys being born has fallen each year in the United States and Japan since the 1970s, according to public health records in both countries.
They say this downward trend may point to the impact of environmental pollutants on male fertility and fetal development -- "a serious matter," the authors note in the study.
Critics, however, say the decrease is only slight and could indicate any number of nonenvironmental factors.
Still, study authors say the numbers should not be ignored -- particularly in light of their finding that an increasing proportion of fetal deaths are male.
Taken together, the study's authors say, the numbers add up to an overall decline of 17 males per 10,000 births in the United States and a decline of 37 males per 10,000 births in Japan since 1970.
This means, according to the authors, that since 1970, 135,000 white males in the United States and 127,000 males in Japan should have been born but were not.
Traditionally, the expected ratio of male to female births is believed to be 1.05. That is, for every 100 girls born, you would expect 105 boys to be born.
The reason for the difference, says Christopher Wills, professor of ecology, behavior and evolutionary biology at the University of California at San Diego, is that Mother Nature stacks the deck in favor of male births. Nature gives males an edge at birth because male fetuses and babies are less hardy than female fetuses and babies. So, by the time males reach the age at which they can reproduce, there should be a one-to-one ratio.
"Generally speaking, males are at a disadvantage compared to females," he says. "In general, as a species, you would want to end up with a roughly one-to-one ratio. The only way to do that is to start out with a much more skewed ratio, and that's what Mother Nature has done."
In 2002, however, this number was 104.6 for babies born in the United States.
Though it is a seemingly small decrease from the expected norm, its implications could be much larger, says lead investigator Devra Lee Davis, director of the University of Pittsburgh Cancer Institute's Center for Environmental Oncology.
Davis says the decline could be due to environmental factors, such as prenatal exposure to certain environmental pollutants. These chemicals, she says, could be disrupting the hormone balance of both fathers and fetuses, leading to changes in the SRY gene -- a sex-determining gene on the Y chromosome that determines the sex of a fertilized egg.
She says such a downward trend, when considered alongside findings in other smaller studies, paint a broader picture of what pollution is doing to our ability to reproduce.
"What we do know is that studies have been done in highly exposed work forces, and in these groups, men are sometimes rendered sterile," she says. "But in some cases, they produced children but only girls. The question is, what explains this?"
"There is something that is affecting our fertility now that has an important effect on the number of males being born," says Lovell Jones, co-investigator and director of the Center for Research on Minority Health at the University of Texas M.D. Anderson Cancer Center.
"The trend was first observed a couple of years ago, and people thought it was a quirk then," he says. "As we've gone back and looked at the records, indisputably, the trend has continued."
However, the Centers for Disease Control and Prevention says the downturn in the ratio of boy babies to girl babies is interesting, but not a cause for alarm.
T.J. Mathews, a demographer with the CDC's National Center of Health Statistics, says that even though a general downward trend can be seen over the decades, it is very gradual -- particularly when the trends before the 1970 cutoff are considered.
Indeed, the graph that illustrated the year-by-year fluctuations in the ratio of boy births to girl births from 1940 onward looks a bit more like the jagged trace of a seismograph than a smooth downward slope.
Follow the average movements, and there is a subtle downward trend. But Mathews says chalking up this downturn to environmental pollutants alone could be jumping the gun.
"We continue to observe these changes, and we have data through 2004," he says. "The thing to keep in mind is that this transition from 1,052 to 1,048 [the sex ratio of babies born from 1940 to present] is not a large one, no matter how you cut it.
"It is not a big change."
In 2005, Mathews wrote a CDC report on the same U.S. numbers seen in the new study. In his report, he concluded, "In the past 60 years the sex ratio has risen and fallen repeatedly, such that short-term comparisons provide little useful information about long-term trends and changes in the sex ratio."
However, Davis says the decreasing sex ratio may indicate other effects that the environment is having on male reproductive health, especially when considered alongside increases in testicular cancer and decreases in average sperm count among males.
"We are now seeing a 30-year trend, and I don't think that 30 years can be dismissed," Davis says. "I think it is arrogant and foolish to try to explain this away."
If there is any common ground between the study's authors and critics, it is that the potential impact of environmental pollutants may play some part in changes in birth trends.
"Our data does not have anything along the lines of determining factors, but there are studied factors that determine that, and environmental toxins are one of them," the CDC's Mathews says.
But he adds that many other factors may be at play, including the age of the parents, obesity of mothers, cultural norms and practices -- and even widespread emotional stress.
"It's an interesting paper," Wills says. "But the number of reasons that this could be happening are immense. I don't think that the paper demonstrates that."
Still, the study's investigators say the threat of pollution to birth ratios is real.
"It is time for us to really change our approach to science," Jones says. "If not, 100 years from now, or even sooner, we may have to talk about the male-female ratio being at such a point that it might affect society.
"There is enough data out there now to say that this issue is something that we need to be concerned about."
"I think that men and women who are thinking about having children need to pay a lot more attention to the things around them in the environment," Davis says, adding that private sector industries must also be conscientious about their impact on the environment.