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Motor vehicle crashes during pregnancy

The Issue

About 1 in 50 pregnant women will be involved in a motor vehicle crash during the average pregnancy in Canada.

These events sometimes require complicated emergency medical care and place mother and baby at risk of death or disability. ICES scientist Dr. Donald Redelmeier, a specialist in the areas of clinical medicine and cognitive psychology, has a distinct interest in traffic safety and examined the risk to mother and child from motor vehicle crashes during pregnancy in two studies.

Pregnant women often worry about air flights, scuba diving, hot tubs and other topics in maternal health, yet may overlook that traffic crashes pose greater health risks.

Dr. Donald Redelmeier

The Evidence

Pregnant women are at increased risk of motor vehicle crashes in their second trimester

Using linked data housed at ICES, Dr. Redelmeier and colleagues Sharon May, Deva Thiruchelvam and Dr. Jon Barrett analyzed the health records (including emergency care visits) of over 507,000 women aged 18 and older who gave birth in Ontario between April 2006 and March 2011. The study looked at the women’s rates of serious car crashes as drivers in the months before, during and after pregnancy. The study design was unique in that the women served as their own controls.

Before pregnancy, the annualized crash rate was about 4.55 per 1,000 women. This risk remained about the same during the first trimester of pregnancy (4.33 events per 1,000 women). In the second trimester, the crash rate increased to 6.47 events per 1,000 women. The crash rate declined during the third trimester to 2.74 events per 1,000 women, and declined further in the year following delivery to 2.35 events per 1,000 women. Few women (less than 0.1%) had more than one crash during the five-year observation period.

Women in their second trimester of pregnancy were 42% more at risk of a serious traffic crash compared to the same women during the same months before pregnancy. The increased risk amounted to an extra 75 crashes sending pregnant drivers to an Ontario emergency department each month. The risk was distinctly high in the afternoon and extended throughout the year. Pregnant women were at increased risk regardless of their age, income, maternal experience or whether they had a standard or longer pregnancy. The risk was about twice the overall population average but still below that of men of the same age.

Dr. Redelmeier wondered if the increased risk of a crash might potentially be related to normal physiological changes that women experience during pregnancy. These changes can include fatigue, insomnia, back pain, neck stiffness, nausea and stress. These changes may increase the likelihood of human error that subsequently contributes to a traffic crash.

The study did not consider the frequency of trips taken during the second trimester compared to other times, the distance travelled or the amount a person drove. Motor vehicle crashes before pregnancy may have been underreported given that worry about the unborn baby was not at issue; pregnant women may be more likely to visit the emergency department even if not badly injured. The study did not track crashes that resulted in fetal death or women whose care was provided by a midwife.

This is not a reason to stop driving. It’s a reason to drive more carefully. In terms of absolute risk, it’s still far more dangerous to be driven by a man of the same age.

Dr. Donald Redelmeier

Motor vehicle crash during pregnancy may increase cerebral palsy risk to unborn child

Dr. Redelmeier’s team then studied how a mother’s motor vehicle crash during pregnancy might possibly affect the long-term health of her child, in particular, whether the blunt injury might be one of many contributors to cerebral palsy. Cerebral palsy is a permanent movement disorder caused by damage to the brain during fetal development, around the time of birth or in the early years of infancy.

Using multiple databases at ICES, the researchers analyzed over 1.3 million children born in Ontario between April 2002 and March 2012 and tracked each for three years to determine a subsequent diagnosis of cerebral palsy. They found that 7,933 children were involved in a motor vehicle crash during pregnancy. Among all the children born, 2,328 were diagnosed with cerebral palsy by age 3 years.

While cerebral palsy is a rare outcome, being in a motor vehicle crash during pregnancy nearly doubled a woman’s risk that her unborn child would be diagnosed with the condition by the age of three if the pregnancy was followed by preterm labour. The absolute risk of cerebral palsy following a preterm pregnancy was 7.3 per 1,000 if no crash occurred and 18.2 per 1,000 if a crash did occur. Children born at term despite a motor vehicle crash had no statistically significant increased risk of cerebral palsy.

The researchers suggested two reasons why a motor vehicle crash might harm a fetus:
  • the sudden impact and deceleration may be similar to a concussion that directly damages brain tissue;
  • blunt forces might damage the placenta and deprive the fetus of nutrients needed for development.

Our data suggest a traffic crash in pregnancy can lead to an increased risk of cerebral palsy in infancy for cases where the woman subsequently had preterm labour. No increased risk was apparent for cases followed by a term delivery. 

Dr. Donald Redelmeier

Recommendations for Action

The researchers recommend that pregnant women, particularly those in the second trimester, avoid excessive speed, obey traffic signs, minimize distractions and always wear a seatbelt. While a motor vehicle crash is not unique to pregnancy, the stakes are higher for the unborn child.

There is a specific role for traffic safety reminders as a part of routine prenatal care. To that end, the Society of Obstetricians and Gynaecologists of Canada should consider incorporating road safety advice into prenatal care guidelines.

Additionally, Ontario should start to track the number of pregnant women involved in traffic crashes; this statistic is not currently compiled by Transport Canada.

Redelmeier DA et al. Pregnancy and the risk of a traffic crash. CMAJ. 2014; 186(10):742-50. <Paper>

Vogel L. Pregnancy and crash risk study wins Bruce Squires Award. CMAJ. 2016;  188(4):249. <News>

Redelmeier DA et al. Motor vehicle crashes during pregnancy and cerebral palsy during infancy: a longitudinal cohort analysis. BMJ Open. 2016; 6(9):e011972. <Paper>

Scientist Profile

Donald Redelmeier

Donald Redelmeier, MD, MSHSR, FRCPC, FACP, is a professor in the Department of Medicine at the University of Toronto where he holds the Canada Research Chair in Medical Decision Sciences. He is a senior scientist at the Institute for Clinical Evaluative Sciences and the Sunnybrook Research Institute and a staff physician at Sunnybrook Health Sciences Centre. Dr. Redelmeier can be contacted at
ICES: Data Discovery Better Health
Established in 1992, the Institute for Clinical Evaluative Sciences is a not-for-profit research institute encompassing a community of research, data and clinical experts, and a secure and accessible array of Ontario’s health-related data.
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