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Mice born by cesarian delivery, or C-section, gained on average 33 percent more weight in the 15 weeks after weaning than mice born vaginally, with females gaining 70 percent more weight. This is the finding of a study led by researchers from NYU School of Medicine and published online on October 11 in Science Advances.

The study revolves around the microbiome, the set of bacterial species living in the human gut. Such microbes evolved over millions of years to play roles in human immunity, digestion, and metabolism. But, they have been disrupted by practices like antibiotic treatment and C-section delivery in recent decades—just as the rate of has more than doubled.

“Our study is the first to demonstrate a causal relationship between C-section and increased body weight in mammals,” lead study author and microbiologist Maria Dominguez-Bello, PhD, tells The Economist. “The question of whether a baby’s founding microbiome affects its future obesity risk becomes more urgent as C-sections are increasingly used by choice in many parts of the world,” says Dr. Dominguez-Bello.

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