Maternal lead exposure increases chance of male offspring

Higher lead levels in a mother’s blood can increase the chance of her bearing male offspring, according to new research led by Tohoku University. The new study, published in the journal Science of The Total Environment, aimed to help explain an observed decline in the proportion of male to female births in Japan and elsewhere.

“Generally, for every 100 females, 104–107 males are born,” said Associate Professor Nozomi Tatsuta, lead author on the study. “In recent years, the sex ratio has been declining worldwide, and the number of male births has been decreasing.”

In demographic terms, the number of males per 100 females at birth is called the secondary sex ratio (SSR), which is known to be sensitive to certain environmental toxins. As noted by study co-author Professor Kunihiko Nakai, “Previous studies have reported that the sex ratio is affected by exposure to chemical substances such as dioxins, as well as heavy metals such as methylmercury.”

Noting that lead can reduce female fertility as well as male sperm quality, the researchers hypothesised that it could play a role in the SSR. The study recruited pregnant female participants from the ongoing Japan Environment and Children’s Study (JECS), which covers a broad geographic swathe of Japan.

The participants were classified into five groups according to their blood lead levels. The analysis was adjusted for family income and smoking status during pregnancy, which could also affect the SSR. Blood samples were collected from 85,171 female participants during middle–late pregnancy and analysed for lead concentrations.

Several analyses were used to examine the relationship between maternal lead levels and SSR — a logistic regression, a sensitivity analysis and a binomial distribution analysis — and all three indicated an increase in the proportion of male births with higher maternal lead concentrations. The correlation between maternal lead exposure and sex ratio was consistent even after adjusting for other potential variables like father’s age and exposure to lead on the job.

The findings may explain why the percentage of male births in Japan has been declining. The authors noted that, as restrictions in leaded gasoline and paint have been implemented, air lead levels have decreased, as have blood lead levels. The study also has implications for public health: guidelines for pregnant women are that blood lead levels should not exceed 50 nanograms per gram of blood, yet the study indicates that a maternal blood level of less than 1 nanogram per gram could affect the sex ratio of offspring.

Since there are many factors other than lead exposure that are related to the sex ratio, the researchers have called for further study of the correlation between lead and offspring sex ratio. Other factors they hope to examine in the future include the effect of paternal blood lead levels on sex ratio and the impacts of lead on frequency of miscarriages and stillbirths.

The authors also caution that because lead can have toxic effects on a developing human brain, it should never be used as a means of trying to control the sex of offspring.

Image credit: ©stock.adobe.com/au/Halfpoint

Please follow us and share on Twitter and Facebook. You can also subscribe for FREE to our weekly newsletters and bimonthly magazine.