Car crash risk during pregnancy

19 May 2014

Interview with

Don Redelmeier, University of Toronto

Pregnancy is one of the most exciting, but potentially also one of the most dangerous times in a woman's life. But it's not just the threat to her health that comes from the physical demands of having a baby. It turns out that women in the middle parts Photograph of abdomen of a pregnant womanof their pregnancies - the so-called second trimester - are much more likely to be involved in life-threatening car accidents.

In fact, according to a new study looking at over half a million Canadian women, the risk of ending up seriously injured in hospital following a collision may be over 40% higher during pregnancy. Don Redelmeier, from the University of Toronto, led the study...

Don - Pregnant women often ask me the strangest questions about scuba diving, roller coasters, airline flights, and even grizzly bear attacks. They almost never ask me about the everyday risks. I was never once asked about road safety despite of being a much larger risk.

Chris - So, you were interested in addressing the fact that there is very, very little data actually looking at whether being pregnant does affect her own safety.

Don - Absolutely right. Standard prenatal care guidelines are virtually silent about the importance of road safety and that includes guidelines in the United Kingdom the United States, Australia, and Canada. We thought that that was sort of a major omission in the existing medical literature.

Chris - So, what did you do to try to fill that gap? How did you approach that?

Don - We identified every woman in Ontario, Canada who gave birth during a 5-year span and we tracked each person for any emergency department visit related to a road crash that occurred either before, during or after pregnancy. So, that amounts to a sample size of over half a million women and about 8,000 life-threatening motor vehicle crashes.

Chris - And that study design inevitably means that because you've got a person who is pregnant when they're having a crash at some points are not pregnant and they might be having a crash, you can directly compare their vehicle accident risk when they are and aren't pregnant.

Don - Exactly right, Chris. It's a huge strength of the study and that each individual serves as their own control. And thereby, we remove confounding from genetics or personality, education, lifestyle, or all sorts of other unmeasured factors that could contribute to roadway risks.

Chris - So, when you do the comparison and ask, is someone more likely to have a crash when they're pregnant compared with when they're not, what trends emerge?

Don - Our largest single finding was that we found that the second trimester of pregnancy led a to a 42% increase in the risk of a serious motor vehicle crash compared to the very same months for the very same person before they were pregnant.

Chris - Do you have any idea as to why you should see that profound escalation in risk and are the numbers big enough to support? Is that statistically and clinically significant finding?

Don - It's an extraordinarily strong finding from a statistical point of view. It's far beyond any explanation related to random chance. One possible theory is that the increase in risk related to a combination of fatigue, insomnia, anxiety, nausea, distraction, excitement, that are all parts of a normal pregnancy and yet, could contribute to driver error.

Chris - Why do you think there is that hotspot in the second trimester of pregnancy? Why not right at the beginning or people might think more obviously, at the end of pregnancy when people are really quite encumbered by - in some cases, a very big belly and some cases, with twins in there?

Don - I think that the extenuation in the middle trimester occurs because that's when women are usually feeling at their best. That's when they're trying to make up for lost time in a first trimester as well as get everything done possible before the baby comes. So that there's a lot of rushing around and there's also potentially a false sense of security in the middle months that needs individuals to potentially lower their guard.

Chris - And when you look at the sorts of accidents that the women are involved in, is there any sort of blame attributed there? Can you see whether they're no claim or it's that person's fault or it was the other person's fault? Who is causing these accidents?

Don - We don't do any at fault analysis. So, it might be that their pregnancy merely impairs a woman's ability to avoid the motor vehicle collision that was setup by somebody else.

Chris - It's interesting because people have looked at people's cognitive performance during pregnancy. They've even looked at people's driving performance all be it using a simulator during pregnancy and they haven't seen these trends or something that would produce such a big increase in risk, but you see. So, how do you account for that?

Don - The past literature about baby brain is really quite controversial, Chris. In community surveys, about 50% of women will describe intermittent episodes of forgetfulness where they misplaced their wallet or they forget about an important appointment. And yeah, most laboratory studies have not been able to replicate large changes in psychology performance. A limitation of those data though is that they're often based on small samples of individuals engaged in artificial tasks and sort of hypothetical risks. So that the strength of our study is, we look at real world driving over an enormous population without any selection bias.

Chris - Do you think that what you're seeing on the roads could be a proxy for a more general phenomenon? Do you think that there could be more general advice given to women in the second trimester of their pregnancy that they are more likely to have other sorts of lapses or accidents and therefore, they need to be more comprehensively careful?

Don - We looked at that pretty carefully in terms of involvement as pedestrians and we didn't find any increase in risk. We didn't find any increase in falls or poisonings, or burn incidents. So instead, the increase in risk is solely isolated to high speed traffic where a small lapse of attention, 1/10th of a second could lead to irreparable consequences.

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