ROUGHLY one human pregnancy in ten presents complications (for example, breech presentation) that might justify the baby being delivered by Caesarean section. In some places that is not possible, and mother and infant have to take their chances with a normal delivery. But the opposite is also true. Elective Caesarean is becoming more and more common. In Brazil, Italy and Iran more than 40% of children are born this way.

That a stressful and expensive procedure is being conducted more often than is strictly necessary has long been a concern. But, more recently, a second worry has emerged. This is that Caesarean section stops infants picking up, from their mother’s vaginas and perineums, bacteria that would normally establish themselves in a newborn’s gut, and by doing so improve its future health. Accumulating evidence suggests three things, in particular. These are that Caesarean babies are more prone than others to allergies (in which the immune system responds to inappropriate stimuli, such as nut proteins), to autoimmune diseases (in which the immune system attacks body cells, as happens in type-1 diabetes), and that they are also more likely to become fat. A study published in Science Advances this week, by Maria Dominguez-Bello at New York University School of Medicine, speaks to the latter hypothesis.

A connection between obesity and the types of bacteria living in someone’s gut is well established, and thus seems the likely explanation for the link between Caesareans and obesity. But this could come about in one of two ways. Either the procedure itself keeps baby and bacteria apart or the large amounts of antibiotics which usually accompany the surgery are responsible. Since testing the distinction on people, though easy, would be unethical, Dr Dominguez-Bello turned instead to mice. She permitted some pregnant rodents to give birth naturally, while performing antibiotic-free Caesareans on others. She then raised the pups in identical conditions.

Her sample was not large—a mere 13 pregnant females produced 69 offspring, of which 35 were born naturally and 34 were delivered by Caesarean. But her results were conclusive. At 15 weeks of age, pups that had been delivered naturally weighed an average of 39 grams. Their Caesarean-delivered kin averaged 45 grams. The probability of this difference resulting from chance is less than one in 1,000. Moreover, when Dr Dominguez-Bello examined the gut bacteria of her mice she found that those born naturally had a normal mixture while those born via Caesarean lacked Bacteroides, Ruminococcaceae and Clostridiales. These are all groups associated with lean bodies.

It seems, then, that in mice—and by extension presumably in women—it is the operation itself rather than the associated antibiotics that are promoting bacteria-mediated obesity. Fortunately, as Dr Dominguez-Bello points out, this should be an easy problem to fix. She is now experimenting with taking a swab of the mother’s vagina and wiping it on an infant’s face shortly after a birth by Caesarean, to try to pass on the relevant bugs. It might sound distasteful. But if it works it will give Caesarean babies a better start in life.