The number of ultrasounds administered to pregnant women has soared over the past decade, new research suggests.

A new study finds that the annual number of ultrasounds rose to 3,264 per 1,000 pregnancies in 2006 from 2,055 per 1,000 in 1996.

The proportion of pregnancies with at least four ultrasounds in the second or third trimesters grew to 18.7 per cent in 2006 from 6.4 per cent in 1996. Women who were deemed low-risk by their physicians received more ultrasounds than women categorized as high-risk.

Women fell into a high-risk category if the pregnancy endangered their life, required a genetics consultation or amniocentesis, or if they had a history of complications in a previous pregnancy. All other pregnancies were considered low-risk.

The study of 1,399,389 single deliveries was conducted between 1996 and 2006 by researchers at McMaster University in Hamilton, Toronto's Institute for Clinical Evaluative Sciences, and St. Michael's Hospital in Toronto. It is published in the Jan. 4 issue of the Canadian Medical Association Journal.

"Although guidelines generally recommend that two ultrasound examinations be performed in a pregnancy without complications — one in the first trimester for measurement of nuchal translucency to screen for aneuploidy (a chromosomal abnormality), and one in the second trimester to screen for fetal anomalies — it is conceivable that the proliferation of prenatal ultrasonography reflects changes in maternal risk over time," write the authors.

The authors speculate that many factors could be involved in the spike in ultrasounds. They attribute it to "defensive medicine," meaning doctors are erring on the side of caution, the desire of physicians to reduce patient anxiety, requests from mothers and even the "entertainment value of seeing one's fetus."

Over the course of the 10-year study period, the proportion of women aged 34-54 rose to 20.4 per cent of all pregnancies from 15.1 per cent in 1996. The number of high-risk pregnancies also grew to 19.3 per cent from 15.7 per cent.

The study's authors also point out that the cost of administering ultrasounds to women at low risk of pregnancy complications is high. At $64 per exam in Ontario, the cumulative total of ultrasound exams in the province since 1996 is $30 million.

"Given the high aggregate costs of prenatal ultrasonography and the evidence of potential overuse in populations not at high risk, health policy makers could make a legitimate argument that costs be contained in groups for whom there is no documented benefit," the study reads.

It also questions the safety of multiple ultrasounds, noting that some studies have shown that frequent scans may cause the fetus's growth to be restricted, delayed speech and non-right-handedness. As well, the study points out that benign findings on ultrasounds can lead to invasive and potentially risky procedures that are not necessary, such as amniocentesis.