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Babies born by in vitro fertilization — when a woman’s egg is fertilized outside of her body and then implanted back into her uterus — skew more heavily male than babies conceived naturally. A study published Monday in the Proceedings of the National Academy of Sciences sheds light on why that happens.

You’ll want to know:

During normal development, female embryos usually shut down the activity of one of their two X chromosomes to keep gene expression levels the same as in male embryos. But researchers at the China Agricultural University saw that this process often went awry in mouse embryos created by IVF. Those female embryos were defective and didn’t develop properly. Researchers said it might be possible to fix the problem by adding a particular chemical (a derivative of vitamin A) to embryos in the lab dish before they’re implanted.

The background:

More than 5 million people have been born using IVF since it was pioneered in 1978. Skewed sex ratio has been debated for decades; one 2010 study found IVF produced as many as 128 boys born for every 100 girls.


Keep in mind:

The study was carried out on IVF embryos in mice, with naturally conceived mouse embryos as the control group. While mice are a commonly used research subject, findings in rodents don’t always translate directly to the biology of humans.

Also note that IVF births account for just 1 to 2 percent of total births in the United States, so having more males born via fertility treatments is unlikely to affect the gender ratio of babies nationwide. “I don’t think anyone should be worried even if there’s a slight increase in male births in patients undergoing fertility care,” said Dr. Mark Sauer, an obstetrician and gynecologist at Columbia University Medical Center, who was not involved in the study.


The bottom line:

Scientists might have figured out the cause for the skewed sex ratio that shows up in IVF births, but it’s not clear yet whether the strategies they propose to fix the issue will work in practice.

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