Women who are married suffer less partner abuse, substance abuse or post-partum depression around the time of pregnancy than women who are cohabitating or do not have a partner, a new study has found.

Unmarried women who lived with their partners for less than two years were more likely to experience at least one of the three problems. However, these problems became less frequent the longer the couple lived together.

The problems were most common among women who were separated or divorced, especially if the couple parted less than 12 months before their child was born.

Dr. Marcelo Urquia, an epidemiologist at the Centre for Research on Inner City Health at St. Michael's Hospital, said that as more children are being born to unmarried parents, he wanted to delve deeper into the risks and benefits of not just single vs. cohabitating parents but the various kinds of relationships.

The results of his study were published today in the American Journal of Public Health.

"What is new in this study is that for the first time we looked at the duration of unmarried cohabitation and found the shorter the cohabitation, the more likely women were to suffer intimate-partner violence, substance abuse or post-partum depression around the time of conception, pregnancy and delivery," Dr. Urquia said. "We did not see that pattern among married women, who experienced less psychosocial problems regardless of the length of time they lived together with their spouses."

Dr. Urquia said knowing the differences between married and cohabitating partners was important as the number of children born outside marriages rises. Thirty per cent of children in Canada are born to unmarried couples, up from 9 per cent in 1971. In several European countries, births out of wedlock outnumber those to married couples.

Dr. Urquia found about one in 10 married women (10.6 per cent) suffered partner or substance abuse or post-partum depression in his study of data from the 2006-07 Canadian Maternity Experiences Survey, a nationwide sample of 6,421 childbearing women compiled by the Public Health Agency of Canada.

He found 20 per cent of women who were cohabitating but not married suffered from at least one of those three psycho-social conditions. The figure rose to 35 per cent for single women who had never married and to 67 per cent for those who separated or divorced in the year before birth.

Dr. Urquia said it was unclear whether problems such as partner or substance abuse were the cause or result of separations.