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Pregnant women worry about everything from airplane travel and hot tubs to cold cuts and rollercoaster rides, but there’s a far more dangerous threat to the health of the unborn child –one that has flown almost completely under the radar until now.

New research by a U of T medical researcher shows that women in their second trimester of pregnancy have a 40 to 50 percent higher risk of getting into a serious motor vehicle crash than they would if they were not pregnant. In Ontario alone, that’s 75 serious crashes a year – almost all of them easily preventable.

“I was surprised by the magnitude of it,” said lead investigator Donald Redelmeier, a professor in the Department of Medicine and the Institute of Health Policy, Management and Evaluation, whose study was published May 12 in the Canadian Medical Association Journal.

“It also surprised me that none of the obstetrical care guidelines mention this in North America, Australia or Europe. They talk about blood pressure and diabetes. But they’re silent about the risks of road crashes.”

Researchers looked at half a million Ontario women who gave birth between 2006 and 2011, and found almost 8,000 had been in serious crashes that sent them to the hospital. That was 42 percent higher than the number of crashes among the same group of women during the same months before they became pregnant.

Young male drivers, however, remain the most prone to accidents, with risks more than 100 percent higher than the population average.

Redelmeier, a staff physician at the Sunnybrook Health Sciences Centre, became curious about crashes after treating a pregnant woman following a serious car accident at the Toronto hospital, which boasts Canada’s largest trauma centre.

“When a crash involves a pregnant driver, it’s extremely challenging to deal with,” he said. “Everything is much more complex, and emotions run high. And then, if anything goes wrong, there’s so much second guessing after the baby is born. You’re left wondering for a lifetime about whether it was the crash that contributed to the unfortunate outcome. It tears at your heart.”

All the usual side effects of pregnancy – exhaustion, nausea, sleeplessness – are very likely behind the increased risk, said Redelmeier, a senior scientist at the Institute for Clinical Evaluative Sciences (ICES).

“If you’re travelling at typical highway speeds, even one-tenth of a second of inattention from feeling exhausted or nauseous, and your car has travelled three meters,” he said. “In that tenth of a second, you use up all the space that would be the difference between a fatal crash and a close call that causes no damage whatsoever.”

He said avoiding these high-stakes risks is almost always as simple as driving more carefully and obeying the traffic rules: driving the speed limit, signaling turns, yielding the right of way, obeying stop signs, minimizing distractions and always wearing a seatbelt. The same advice is important for other drivers who share the road with pregnant women.

Redelmeier hopes his discovery will prompt physicians to make road safety warnings a routine part of prenatal care.

Heidi Singer is a writer with the Faculty of Medicine at the University of Toronto.