Researchers at the University of Toronto reported today in the Canadian Medical Association Journal (CMAJ) that pregnancy is associated with a considerably higher risk of serious motor vehicle crash, particularly among drivers in their second trimester compared with their risk before pregnancy. The authors of the report defined serious motor vehicle crash as one in which the pregnant driver was sent to a hospital emergency room for suspected or apparent injuries. This risk of serious crash in the first month of the second trimester peaked at 42% and dropped precipitously by the third trimester.

“It amounts to about a one in fifty statistical risk of the average women having a motor vehicle crash at some point during her pregnancy,” said study leader Donald Redelmeier of the University of Toronto.

The longitudinal cohort study involved a total of 507,262 Ontario women over 18 years of age who gave birth during the study period that ran between April 1, 2006 and March 31, 2011. The study population when behind the wheel accounted for 6922 crashes during the study’s three-year baseline period, which calculates to a rate of 177 crashes per month. When behind the wheel during their second trimester of pregnancy, these women accounted for 757 crashes or 252 crashes per month. When compared to the baseline crash rate, the second trimester drivers had a relative risk of 1.42, meaning that the probability of crash for drivers in their second trimester was 1.42 times higher than their probability of crash before pregnancy.

The research team observed that the increased risk “extended to diverse populations, varied obstetrical cases, and different crash characteristics.” Their analysis revealed that the relative risk for crash while pregnant was higher among urban versus rural female drivers and slightly higher for women with prior births compared with women who were experiencing their first pregnancy. Relative risk of crash for these women was also higher in the morning. Relative risks were not elevated for the women when they were passengers or pedestrians.

The researchers speculated little on reasons for why the risk for crash is so high among the second trimester, citing a paucity of research into possible cognitive lapses during pregnancy.

“The message here is not to stop driving,” said Redelmeier. “The message is to start driving more carefully.”

The CMAJ has made the full-text report available for download.