Pregnancy and risk of a traffic crash
=====================================

* Donald A. Redelmeier
* Sharon C. May
* Deva Thiruchelvam
* Jon F. Barrett

McKillop1 emphasizes that motor vehicle crash risks can vary depending on the weather, and that adverse weather is correlated with the Canadian winter. We agree, and this was our rationale for a secondary analysis comparing each individual’s middle trimester of pregnancy to the same months exactly a year earlier. This analysis yielded a somewhat higher relative risk associated with pregnancy (odds ratio = 1.62, 95% confidence interval 1.45–1.82, *p* < 0.001). Evidentially, our findings cannot be attributed to seasonal fluctuations in adverse weather.2

Buchan and collegues3 suggest that a woman’s increased propensity to seek care while pregnant might bias our analysis. We tested this idea by examining three measures of crash severity during the middle trimester and baseline interval. We found that the frequency of a higher emergency triage score was increased for crashes during pregnancy (74% v. 58%, *p* < 0.001). The frequency of ambulance involvement was about the same (47% v. 45%, *p* = 0.308) as was the frequency of admission (2.6% v. 3.2%, *p* = 0.409). An increased propensity to seek care is unlikely to explain the magnitude of our findings.

Each letter1,3 also motivates points of clarification. McKillop1 is not perfectly correct in claiming that crashes occur more often in winter — only property damage crashes increase in the winter. Crashes that cause serious injury are more common during the summer.4,5 Buchan and colleagues3 are not perfectly correct in implying that crashes can be innocent exposures for pregnant women; instead, low-impact speeds can lead to placental separation, uterine rupture or diffuse axonal damage in the fetus. Case studies in obstetrics indicate that amniotic fluid does not perfectly protect a fetus in a crash.6,7

## References

1.  McKillop JM. Pregnancy and risk of a traffic crash [letter]. 2014;186:1169.
    
    

2.  Redelmeier DA. The exposure–crossover design is a new method for studying sustained changes in recurrent events. J Clin Epidemiol 2013;66: 955–63.
    
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3.  Buchan S, Pulver A, Ray JG. Pregnancy and risk of a traffic crash [letter]. 2014;186:1169.
    
    

4.  Redelmeier DA, Drucker A, Venkatesh V. Major trauma in pregnant women during the summer. J Trauma 2005;59:112–6.
    
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5.  Ontario road safety annual report (ORSAR) 2010. Toronto: Ontario Ministry of Transportation; 2010.
    
    

6.  Hayes B, Ryan S, Stephenson JB, et al. Cerebral palsy after maternal trauma in pregnancy. Dev Med Child Neurol 2007;49:700–6.
    
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7.  Weiss HB, Sauber-Schatz EK, Cook LJ. The epidemiology of pregnancy-associated emergency department injury visits and their impact on birth outcomes. Accid Anal Prev 2008;40:1088–95.
    
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