C-section babies have a different microbiome - but not for long

Babies born by caesarean section have different gut bacteria to those born vaginally – but the differences largely disappear by the time the babies are between 6 and 9 months old. That’s according to the largest study into the effects of birth mode on the microbiome.

Previous research has suggested that babies born by C-section are more likely to collect hospital-acquired bacteria when they are born, while those born vaginally collect microbes from their mother. Because the gut microbiome is thought to be intricately linked to health, these differences have been suggested to make C-section babies more likely to develop obesity, asthma and eczema.

“The hypothesis is that the moment of birth might be a sort of thermostatic moment for the immune system… that sets the immune system for future life,” says Nigel Field at University College London.

To investigate the phenomenon, Field and his colleagues collected faecal samples from 596 babies born in UK hospitals. The samples were collected by the babies’ parents over the first few weeks of life, and when the babies were between 6 and 9 months old. “We did quite a bit of experimenting to show that baby poo is pretty stable in the post for a couple of days,” says co-author Peter Brocklehurst at the University of Birmingham, UK.

The team found that about 80 per cent of C-section-born babies had hospital-acquired bacteria in their guts when they were born, compared with 50 per cent of vaginally born babies. And the bacteria made up around 30 per cent of the total bacteria in C-section babies, compared with just 10 per cent in babies born vaginally.

But by the time the babies were weaned at the age of around 6 to 9 months, these differences had largely disappeared. And all the babies were healthy, so the researchers can’t tell if there are any implications for health. “We don’t know the long-term consequences of these findings,” says Field.

The findings shouldn’t put expectant mothers off having a C-section, says Alison Wright of the UK Royal College of Obstetricians and Gynaecologists. “Women often need a caesarean, or some women choose caesarean, and although this is ground-breaking research, at the moment this should not deter women from having a caesarean section,” she says.

The team also took faecal samples from 175 mothers and compared them with those of their 178 babies. They were surprised to find that the babies’ gut bacteria seemed to be coming from their mothers’ guts, rather than from the vagina during delivery.

The finding pours more cold water over the practice of vaginal seeding, which has gained popularity in recent years. The idea is to put a swab in the vagina of a woman who is about to have a C-section, then wipe the swab over the baby’s face or mouth to deliver the bacteria it might have picked up in a vaginal birth.

The practice isn’t recommended by any clinical body, because it can be dangerous. The new findings suggest that it would be pointless too – the bacteria picked up during vaginal births seem to come from the anal area rather than the vagina, the team say.