Women That Are Exposed to Lead Are More Likely to Give Birth to Boys

Demography has its own terminology. Among the terms specific to this field of science is sex ratio which indicates the number of males per 100 females at birth. For some years now, this number has been falling. Researchers at Tohoku University in Japan wanted to understand why. In the journal Science of the Total Environment, they explain that these variations in the rate may be related to the environment, and more specifically to the presence of lead in the air.

Baby Boy

Baby Boy

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Lead, harmful to fertility

In general, 104 to 107 males are born for every 100 females,” explains Nozomi Tatsuta, associate professor of medicine and lead author of the study. In recent years, the sex ratio has decreased worldwide and the number of male births has declined. “This ratio may be influenced by several factors, including environmental toxins,” he said. “Previous studies have reported that the sex ratio is affected by exposure to chemicals such as dioxins as well as heavy metals such as methylmercury,” adds study co-author Kunihiko Nakai, professor of environmental medicine. Since lead can reduce both women’s fertility and men’s sperm quality, the researchers speculated that it might play a role in changing the sex ratio.

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The higher the blood lead level, the more male births

More than 85,000 pregnant women participated in the study, which was conducted in Japan. They were divided into different groups according to their blood lead levels, measured during pregnancy. Family income and smoking during pregnancy were taken into consideration when analyzing the data. The scientists used different methods to observe the effects of lead on the sex ratio. In all cases, there was an increase in the number of male births when the blood lead concentration was higher. “The correlation between maternal lead exposure and the sex ratio was consistent, even after adjustment for other possible variables,” say the authors.

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Reduced lead exposure

According to the authors, these results may explain why the male birth rate has declined in Japan: they point out that various measures have been taken to reduce lead exposure, particularly from paint and gasoline. As a result, lead levels in the air have dropped, which could explain why fewer males are being born. Furthermore, these results may have implications for public health recommendations. Japanese scientists note that maternal blood lead levels of less than 1 nanogram per gram already affect pregnancy and may influence the sex ratio of offspring, but current guidelines state that blood lead levels should not exceed 50 nanograms per gram of blood.

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References

Effects of maternal exposure to lead on secondary sex ratio in Japan: The Japan Environment and Children’s Study

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