New mothers should not embrace the trend of "seeding" their babies with vaginal bacteria, say doctors.
It exposes children born by Caesarean section to bacteria that could have coated their bodies if they had been born vaginally.
The idea is bacteria help train the immune system and lower the risk of allergies and asthma.
But doctors in Denmark and the UK said there was too little evidence and it may be doing more harm than good.
Being born by Caesarean section is linked to a higher risk of some immune-based diseases.
And there is growing medical interest in the role of the microbiome - the micro-organisms that call our bodies home - in preventing disease.
Infection risk to babies
Seeding involves taking a swab of vaginal fluid and rubbing it into the newborn's face, skin and eyes.
A report, published in BJOG: An International Journal of Obstetrics and Gynaecology, said more than 90% of Danish obstetricians had said they had been asked about vaginal seeding.
It said there was no evidence of any benefit to seeding as there was only one proper study of the technique and it involved just four babies.
However, it warned of clear risks to the baby, including infections such as group-B streptococcus, E. coli and a range of sexually transmitted infections.
Dr Tine Clausen, the report author and a consultant at Nordsjaellands Hospital in Denmark, said: "We know that women and their partners are increasingly speaking to their doctors about vaginal seeding."
She told the BBC News website: "I really understand, it's a fascinating thought that you're able to mimic nature by doing the seeding, but it's based on some theoretical thoughts and we don't have evidence to support it."
Dr Clausen said a swab may not contain the same bacteria as those transferred during a vaginal birth and any bacteria were more diluted because of blood and amniotic fluid in the vaginal tract during labour.
Her advice to women is to "avoid unnecessary [Caesarean] sections, aim for breast feeding for at least half a year and to have early skin-to-skin contact".
Each of which does have a beneficial impact on a child's microbiome.
In the UK, about a quarter of babies are born via Caesarean section
Dr Patrick O'Brien, from the Royal College of Obstetricians and Gynaecologists, said: "There is no robust evidence to suggest that vaginal seeding has any associated benefits.
"We would therefore not recommend it until more definitive research shows that it is not harmful and can in fact improve a child's digestive and/or immune system."
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