New mothers could be putting their babies at risk of serious infection by following a trend called "seeding", experts have warned.
Doctors said demand for the practice - also known as microbirthing - is rising in UK hospitals but there is no proven benefit for newborn babies.
When babies are born naturally they are exposed to a range of beneficial bacteria while moving down the birth canal.
But babies who are born by Caesarean section are not exposed to this bacteria.
With seeding - which apparently started in Australia but is now growing in popularity in the UK - mothers are requesting that their babies are covered in vaginal fluid immediately after a Caesarean birth via a swab.
Parents hope the exposure to bacteria will boost their baby's immune system, thereby preventing illness and disease in the future, such as asthma and allergies.
But experts writing in the British Medical Journal have warned that new mothers could be putting their babies at risk of harm, including from group B strep (GBS), which is a serious infection that can lead to death.
Around a quarter of pregnant women carry GBS at any one time, with a small number of babies becoming infected during birth.
In their article for the BMJ, Dr Aubrey Cunnington, a clinical senior lecturer from Imperial College London, Aniko Deierl, a consultant neonatologist at St Mary's Hospital, London, and Eimear Brannigan, a consultant in infectious diseases and infection prevention and control at Charing Cross Hospital London, said demand was growing in their hospitals and "has outstripped both professional awareness and professional guidance on this practice".
The team said that in some countries, including the UK, pregnant women are not tested for GBS. While GBS may be transferred during a vaginal birth, a Caesarean would actually provide protection against it and other diseases.
"(GBS) is carried by around one in four pregnant women, and although it poses no risk to the mother it can cause fatal infections in babies," Dr Cunnington said.
"There are also other conditions that cause no symptoms in the mother, such as chlamydia, gonorrhoea and herpes simplex virus that could be transferred on the swab.
"One colleague had to intervene when a mother with genital herpes, who had undergone a Caesarean section, was about to undertake this process. Swabbing would have potentially transferred the herpes virus to the baby."
Dr Cunnington said that often the husband or partner undertakes the swabbing process, but in some cases parents have asked medical staff to perform the procedure.
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