Waves of Canadian women are getting extra ultrasounds during pregnancy, often three or more -- a rate that appears to be climbing for no apparent medical reasons.

A new study based on nearly 1.4 million deliveries in Ontario shows more than a third of women -- 37% -- now receive three or more ultrasounds in their second and third trimesters.

Current guidelines recommend only two ultrasounds for uncomplicated pregnancies.

"While the benefits of prenatal ultrasound in high-risk pregnancies may be more clear, the value of repeat ultrasounds in low-risk patients is not," the researchers report in a study published this week in the Canadian Medical Association Journal.

Ultrasounds are widely regarded as safe. But the scans expose the fetus to a form of energy, "and there remains a theoretical risk for subtle effects on fetal development," according to the Society of Obstetricians and Gynaecologists of Canada.

Moreover, as ultrasounds become increasingly sophisticated, more "incidental" and benign findings are being picked up that turn out to be nothing at all. That can cause anxiety and lead to more, and sometimes invasive tests, such as amniocentesis, where fluid is removed from the sac around the baby to look for birth defects and chromosomal problems. The procedure can lead to miscarriage.

Overall, the study found that the proportion of pregnancies with four or more second or third-trimester ultrasounds increased nearly threefold over the study period, from 6.4% in 1996, to 18.7% in 2006. The increase was more pronounced among low-risk pregnancies than high-risk ones.

The rising rates can't be explained by changes in risks to mothers or babies, the authors say. Rather, it appears non-medical reasons are behind it, including "defensive medicine" and the "entertainment value" of baby pictures.

Most guidelines recommend two ultrasounds be performed in pregnancy without complications -- one in the first trimester that measures the thickness of the skin fold behind the back of the baby's neck to check for such chromosomal abnormalities as Down syndrome, the other, a head-to-toe anatomical scan in the second trimester to screen for birth defects.

The new study was conducted at the Institute for Clinical Evaluative Sciences (ICES) in Toronto. Researchers looked at all in-hospital deliveries from 1996 to 2006, but excluded those for multiple births, such as twins or triplets, "where ultrasounds are used often, for very good reasons," said principal investigator Dr. John You, an assistant professor of medicine at McMaster University in Hamilton and ICES scientist.

Next, they looked at Ontario Health Insurance (OHIP) claims to count the number of ultrasounds per pregnancy, on a year-by-year basis. The rate increased 55% over the 10-year study period.

"A 55% increase is pretty substantial. Most things don't increase by that much over a 10-year period," Dr. You said.

The study used Ontario data only, "but I certainly don't think we've seen a decrease over time in any other provinces," Dr. You said. "I wouldn't be surprised if we saw similar findings across the country."

The finding held even after researchers took the mother's age, rates of diabetes, high blood pressure and other factors into account.

Obstetricians practice in a high-risk malpractice environment; they're sued more often than any other kind of doctor.

"Obstetricians may feel pressure to provide reassurance to their patients with a fairly inexpensive and safe test," You said.

But Dr. You said the downside of so many ultrasounds is cost and anxiety.

"The technology has come along and we're now seeing things on the images that we just didn't see 10 years ago, small blips on the pictures. I think increasingly the problem is that we're detecting things that may not be of any importance to the fetus, or the baby's health, but that can provoke a fair bit of anxiety."

A single ultrasound costs about $65. Nearly 3.6-million scans were done in Ontario over the 10-year study period, totalling an estimated $232-million.

"We're not arguing at all that ultrasounds are being overused in complicated, high-risk situations where there's pretty clear benefit from it," Dr. You said. "But, we're seeing lots of people getting four, five, six ultrasounds" for uncomplicated pregnancies.

Some studies have suggested possible links between frequent ultrasounds and smaller birth size, or babies more likely to be left-handed, which isn't in itself a problem but researchers have wondered what effect frequent scans might be having on the fetal brain. But Dr. You stressed the links are "soft" and not at all proven.

"As far as we know, ultrasounds really are quite safe,"he said. "But since we don't know 100% for sure, there's little reason to do additional ultrasounds if there's not much that can be gained from it."

An official with the Society of Obstetricians and Gynaecologists of Canada said there are risks to extrapolating findings from a large population study to an individual person.

"You cannot practise medicine by saying, ‘look, sorry Mrs. So-and-so, you've had now two or three ultrasounds, I can't do anymore," said executive vice-president Dr. Andre Lalonde.

"Our guidelines are pretty clear. We usually recommend for all pregnancies two ultrasounds. (But) we know there is a large majority of women who will require more."