Previous studies have suggested that the mode of delivery affects the newborn’s microbiome, but some said this might be because many mothers who have had caesarean sections require antibiotics. A study of more than 100 babies suggested that those born by caesarean section had different gut microbiome — and this could affect their health later in life. The new research, which was presented at the European Congress of Clinical Microbiology and Infectious Diseases in Amsterdam, backs up claims that it is specifically the mode of delivery that drives the differences. The team involved in the research collected feces at 10 points in the year from 46 babies born by caesarean section and 74 babies born vaginally, starting from their first feces, and analyzing its microbial makeup. Crucially, the team says that antibiotics, if needed, were only given to mothers once the baby was delivered — meaning that babies were not directly exposed to the antibiotics. The results, based on analysis of stool both from the mothers and the babies, also showed that babies born by caesarean section were slower to acquire certain “good bacteria” that are important in digesting milk, and had higher levels of certain types of potentially harmful bacteria than those born vaginally. “We feel that it is proved that mode of delivery is an important driver or modifier of the gut microbiome in young infants,” said Prof Debby Bogaert from the University of Edinburgh, who worked on the project with colleagues in the Netherlands. She added that the conclusion was backed up by findings that antibiotics given to mothers after delivery did not appear to affect their own gut microbiome, and that differences in babies’ microbiomes were also seen in babies that were only bottle-fed, suggesting the effects were not down to babies receiving antibiotics through breast milk. The team also adds that babies with higher levels of the potentially harmful bacteria tended to have more respiratory infections within their first year. A second study presented at the conference looked at whether the microbes found in babies’ guts were linked to their chance the first stool passed after birth, which is thought to form when the baby is in the womb. The results show that the makeup of the microbiome within a baby’s first stool is linked to whether the child is overweight at three years of age. Korpela, one of the researchers involved in the study from the University of Oulu, said there could be many factors affecting the microbial makeup of a baby’s first stool, including the mother’s use of antibiotics during pregnancy and the living environment of the pregnant mother. However, it is unclear whether the observed link is one of cause and effect, or whether the mode of delivery influences the results. “The concept of fetal microbiome is very controversial and the colonization process after birth is better understood than the colonization process before birth,” said Korpela. But, she said, it was interesting that the microbiome formed before the birth was possibly linked to children’s later health.