Mums-to-be have been warned not to have souvenir scans of their unborn baby in the first 10 weeks of pregnancy.
The Royal College of Obstetricians and Gynaecologists said the growing trend for the keepsake ultrasound images – which uses high-frequency sound waves to photograph the foetus – could expose the baby to unknown risks.
There is no evidence that they actually cause any harm, but RCOG says the 'precautionary principle' should apply.
Dr Christoph Lees, Reader in Obstetrics and Fetal Medicine at Imperial College London, said: "Ultrasound scanning in the embryonic period is an invaluable resource in several important scenarios where the embryo is at possible risk.
"There are presently no grounds for questioning the safety of diagnostic ultrasound in this context.
"However, ultrasound imaging is increasingly being used without obvious medical justification and we have to be aware of the possibility of subtle long-term adverse effects, particularly in the first weeks of gestation when the embryo is potentially the most vulnerable."
The review was produced by the Scientific Advisory Committee at the RCOG to provide doctors with up-to-date information about the issue.
Dr Lees said one of the possible harms might come from the slight heating effect produced by ultrasound which was more easily dissipated by the placenta after 10 weeks of pregnancy.
He said the safest period for taking souvenir scans was 20 weeks of pregnancy and beyond.
Dr Sadaf Ghaem-Maghami, chair of the RCOG's Scientific Advisory Committee, said: "We are adopting a precautionary approach and are highlighting the small but possible risks to women so that they can make informed choices.
"We also emphasise that any healthcare professional involved in the use of ultrasound is aware of the safety principles of ultrasound."
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