Parents-to-be, hoping to get the first close-up look at their growing babies, are being urged to avoid private clinics that offer ultrasound scans for keepsake images or videos.

Over the last two decades several of these clinics have opened up, offering ultrasound images, 3D images, or 3D videos that are described as "4D imaging."

The U.S. FDA says while there has been no evidence that such ultrasounds cause harm to the fetus, it nevertheless "strongly discourages" using the scans for non-medical reasons.

Ultrasound heats tissues slightly, and the long-term effects of such tissue heating are not known, the FDA says in its warning. Ultrasounds can also produce very small bubbles in some tissues, which is called cavitation.

"The long-term effects of tissue heating and cavitation are not known. Therefore, ultrasound scans should be done only when there is a medical need," the agency advises.

In Canada, the Society of Obstetricians and Gynaecologists of Canada (SOGC) and the Canadian Association of Radiologists issued a similar warning against "entertainment" ultrasounds in a joint policy statement issued earlier this year.

They too warned that every ultrasound involves exposes the fetus to "targeted energy," and carries a theoretical risk of some harm.

They also warned that there is no assurance that ultrasound operators at these private clinics have received proper training, or that the equipment has been well maintained or sterilized.

The FDA is also urging parents not to buy and use their own Doppler ultrasound heartbeat monitors.

These monitors are prescription devices and should only be used under the supervision of a health care professional, says FDA biomedical engineer Shahram Vaezy.

"When the product is purchased over the counter and used without consultation with a health care professional taking care of the pregnant woman, there is no oversight of how the device is used," Vaezy said in a statement.

"Also, there is little or no medical benefit expected from the exposure."