Study: IVF Causes Higher Rates of Baby Boys

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Though the ratio of boys to girls born in the United States has been on the decline for decades, new research found that moms who use in vitro fertilization might increase the number of male babies.

The University of New South Wales study, published in BJOG: An International Journal of Obstetrics and Gynaecology, found that IVF treatment raised the sex ratio from 105 boys per 100 girls to 128 boys per 100 girls.  Researchers will need to do further studies to determine why this happens. (More on Time.com: Boy or Girl? Change Your Diet, Micromanage Sex — and Other Pregnancy Myths)

Researchers tracked the live births of women in Australia and New Zealand between 2002 and 2006 who received either IVF or another type of fertility treatment, intracytoplasmic sperm injection (ICSI).  The difference between the two treatments is simple numbers: during IVF, doctors allow many sperm to swim to the egg, while ICSI is the direct injection of a single sperm.

“The results from this study demonstrate that a particular [assisted reproductive technology (ART)] procedure or treatment course can alter the probability of having a male baby from the natural sex ratio at birth in Australia of about 51.5%, to a low of 48.7% (a decrease of 2.8%) or a high of 56.1% (an increase of 4.6%), depending on the procedure used,” said co-author Jishan Dean in a statement. (More on Time.com: What Goes on Inside the Brain of a Misbehaving Boy?)

In vitro fertilization is a process in which a woman’s egg is fertilized outside of her body and then implanted into the uterus after a number of days. Interestingly, the number of days mattered: the ratio of boys to girls grew higher the later the fertilized egg was transferred to the womb. Women who had eggs transferred after four days had boys 54% of the time, whereas eggs transferred at two to three days were 49.9% likely to be boys.

But BJOG‘s editor-in-chief, Philip Steer, cautioned against using these results to attempt to deliberately choose the sex of a baby, citing population imbalances in countries like China and India where male babies are preferred.

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