Babies delivered by Caesarean section may lack some healthy bacteria in their guts, experts suggest.
According to a study, a reason for the lower levels of bacteria is because they are usually transmitted from mother to child during a standard delivery.
Parents are being warned that Caesarean-delivered babies could, therefore, be more prone to allergies.
The report, published in the journal Gut, explains how Swedish researchers examined gut bacteria from 24 babies. Of these, nine were born by C-section at different ages: one week after birth, and then at one, three, six, 12 and 24 months old.
Blood samples were also taken to test the immune response of the babies.
Babies who did not pass down the mother's birth canal - i.e. by Caesarean - lacked the gut bacteria Bacteroidetes, or were delayed in acquiring it when compared with natural births.
Bacteroidetes were not detected until a year later in some C-scetion babies, while overall they had a lower range of other groups of bacteria.
The scientists said: "Microbial colonisation of the infant gut gastrointestinal tract is important for the postnatal development of the immune system.
"In this study, Caesarean section-delivered infants who are not entering the birth canal of the mother either lacked or displayed a delayed colonisation of one of the major gut phylum, the Bacteroidetes.
"The colonisation of this phylum was delayed by up to one year for some infants. The total microbiota diversity was also lower in the Caesarean section infants, probably largely as a consequence of the lack of this phylum."
Copyright Press Association 2013