Expectant women are almost twice as likely to crash their cars than women who aren't pregnant, a study has found.

The Institute for Clinical Evaluative Sciences (ICES) in Canada conducted research involving 500,000 women over a five-year period and found that in the second trimester of pregnancy, the risk of being involved in a car crash rose from 4.55 instances per 1000 females to 6.47 instances per capita. The worst period of all was the fifth month of pregnancy, in which the instance rate rose to 7.66 per 1000.

Of the 507,262 women involved in the research, 6922 were involved in serious car crashes in the three years before falling pregnant, equating to 177 per month. That figure rose to 252 per month for the second trimester of pregnancy.

The risk was lowest in the year following the child's birth, which ICES said reflected the safer driving behaviours and attitudes of new parents.

Lead researcher Donald Redelmeier said the results don't mean pregnant females should be worried about getting behind the wheel - but that they should be aware that in the second trimester, most women will be affected by nausea, fatigue and distraction.