Babies born by elective caesarean are more likely to become obese, according to a new study.
It's suspected to be because the babies are not exposed to key microbes during labour, or the reduced stress of not having to be pushed out of the womb.
Lead author Sir Peter Gluckman, former science advisor to the Prime Minister, says the hours before birth are crucial.
"There's reason to think that what happens at that time may have long-term consequences, and not get reset again."
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Caesarean rates have doubled worldwide in the last 20 years, and more and more of them are elective - planned in advance, and not necessarily required for any medical reason.
In New Zealand, around a quarter of all births are caesarean - more than two-thirds of those elective (17.7 percent of all births). In Auckland, more than a third of all babies are born via caesarean section. In 2006, fewer than 13 percent of births were elective caesarean.
Researchers looked at 727 mother-child pairs, and found elective caesareans were "significantly associated" with the child being overweight by the time they were a year old. Obesity rates didn't rise in children born by emergency caesarean.
But Sir Peter says obesity may just be the tip of the iceberg.
"I don't think the only outcome that's different is going to be obesity. That happens to be something that's easy to measure."
Researchers are urging doctors to discuss the possible implications of caesarean births with expectant parents.
"This rising epidemic of elective caesarean section is not without some potential cost to the baby."
The study was published Thursday in journal JAMA Network Open.