Parents have been warned against the use of private companies to store blood from their babies' umbilical cord.
Cord blood contains stem cells
Thousands of couples have already paid up to £1,500 to store samples which could potentially be used in the treatment of disease.
But a Royal College of Obstetricians and Gynaecologists report said there was "insufficient evidence" to recommend the practice.
Doctors have raised concern that some firms are playing on patients' fears.
Some promote their service to parents through leaflets provided in GPs surgeries and antenatal clinics.
The major use for cord blood stem cells has been in the treatment of cancers such as leukaemia in children.
But some companies claim the stem cells may also be used in the future to treat conditions such as Parkinson's disease.
The NHS collects around 1,700 to 2,000 cord blood samples every year for storage in a public bank that can be used by anyone who needs a cord blood cell transplant.
Cord blood storage is also done in families at high risk of a condition - such as Fanconi anaemia - which could be treated with a transplant.
CORD BLOOD STORAGE
One-fifth of stem cell transplants are done in children and young people with leukaemia
The chance of an individual using personal cord blood for a blood cell disorder before the age of 20 is estimated to be between 1/20,000 to 1/37,000
The NHS cord blood bank has around 7,000 donations
The report from the RCOG advises health professionals how to safely deal with requests from parents for commercial cord blood storage.
Professor Peter Braude, head of the Department of Women's Health at King's College London and Chair of the RCOG expert group, said: "We have had a huge amount of commercial companies offering cord blood storage to parents and medical practitioners wanted guidance on the validity of some of the advertising claims.
"The likelihood of use is extremely small and some of the uses are extremely speculative.
"Most people are unaware there is an NHS cord blood bank and most transplants are done with bone marrow.
"Trusts should have a clear policy about what they will and will not allow.
"We can find no evidence to suggest [private banking] is a particularly good idea."
If a trust decides to let parents collect cord blood to be stored privately, the college recommends that there should be no change in the usual management of birth.
Doctors and midwives should not take part in collection as it could be dangerous to direct their attention away from the mother and baby - instead it should be done by a trained third party.
The committee also called for improved funding so the NHS could store more samples, and maximise access to those in need.
Currently, cord blood collection for the NHS bank is done at three hospitals in London.
'A once in a lifetime opportunity'
'Like freezing a spare immune system'
Slogans used in promotional material by companies offering cord blood storage
There is also a hospital in Newcastle that does collection but is not doing so at the moment because of funding issues.
Dr Ruth Warwick, president of the British Association for Tissue Banking, said: "The NHS cord blood bank was funded with research and development money.
"There is now NHS money coming in but we would like to see more guarantees for the future so it wouldn't be hand to mouth finances."
Professor Braude added: "We would like to see the need [for private banks] is no longer there.
"If we had a large enough NHS bank we wouldn't have private banks because we could get cord blood when we needed it."