Parents-to-be are being urged to be cautious about so-called "boutique ultrasounds" from companies offering scans of their unborn babies.
Many companies offer scans to expectant parents
The British Medical Journal says doctors are worried about the rise in companies offering "keepsake" scans.
There is no evidence ultrasounds have ever caused harm, but the fear is that energy from them could raise the temperature of a baby's tissues.
Ultrasound experts say it should only be used if there is a medical benefit.
Ultrasound scans are a key part of the NHS care a pregnant woman receives, but whereas NHS sonographers who carry out the scans are regulated - although this is not mandatory, there is no regulation of the commercial clinics who offer ultrasounds.
Conventional ultrasounds are two dimensional, and expectant parents take these grainy black and white shots away from their NHS scans.
But commercial companies are now offering 3D images, and even moving images of the child.
Writing in the BMJ, Geoff Watts says expectant parents seeking a CD-ROM or a DVD of their scan can pay between £150 and £250.
The companies that offer these services, widely advertised in pregnancy magazines and on websites, say ultrasound has not been shown to cause any harm to mother or baby.
But medical experts, including the US Food and Drug Administration (FDA) caution "casual exposure" should be avoided.
And there are also worries about how a commercial scanner would deal with finding a medical abnormality in the baby.
The FDA has said: "Although there is no evidence that these physical effects can harm the foetus, public health experts, clinicians and industry agree that casual exposure to ultrasound, especially during pregnancy, should be avoided."
Kevin Martin, president of the British Medical Ultrasound Society, said: "Our view is that ultrasound should only be used by those qualified to do so, who are in a position to ensure it is carried out safely.
"Ultrasound should not used solely for producing souvenir videos in the first trimester because the baby's cells are still differentiating at that stage.
"If there's no obvious medical benefit to the baby at that stage, it should not be put at any risk."
He said that, after the first trimester, only qualified practitioners should perform ultrasounds and the output of the machine should be at a low level.
But Professor Stuart Campbell, who used to carry out ultrasounds in the NHS but now works with Create Health, a private clinic which offers scans, told the BMJ people should not be concerned over the safety of these scans.
He said doctors in training, who have not yet mastered ultrasound techniques, exposed pregnant women to much higher doses of high-frequency ultrasound than a skilled operator would because they take longer.
"If you're competent at what you're doing, the extra five minutes [required to generate a keepsake scan] is absolutely negligible."