Unmarried couples who live together may have a higher rate of pregnancy complications compared with those who are legally wed, according to a Canadian study.

Looking at more than 720,000 births in Quebec between 1990 and 1997, researchers found that so-called common-law couples had “modestly higher” risks of premature delivery, low birth weight and infant death compared with married couples.

The reason for the disparity is unclear, but its persistence throughout the 1990s is surprising, study co-author Dr. Michael S. Kramer of McGill University in Montreal told Reuters Health.

That’s because the province of Quebec, which grants common-law couples many of the same rights that legally married couples have, has seen an explosion in such unions in recent years. In 1998, births to common-law mothers surpassed those to married women living in the province, Kramer and his colleagues note in the journal Obstetrics & Gynecology.

According to the authors, Quebec defines a common-law union as one between two adults who publicly consent to live together and “respect the resulting rights and obligations.”

Given the frequency of these relationships and their growing social acceptance, it’s surprising that these couples consistently showed worse pregnancy outcomes over time, according to Kramer.

It’s possible, he noted, that in some cases cohabitation is a sign of a less stable relationship, and that this could affect the pregnancy. However, Kramer said, more research is needed to figure out the reason for the disparity between married and unmarried couples.

During the study period, the proportion of births to common-law couples shot up from 20 percent in 1990 to 44 percent in 1997. Overall, 53 percent of births during this time were to legally married women, 35 percent to women in common-law unions, and the rest to single mothers.

While pregnancy outcomes were somewhat poorer for common-law couples than for married ones, they were “much better” than those of single women, Kramer and his colleagues report.

Compared with legally married couples, cohabiting ones were 14 percent more likely to have a premature baby and 21 percent more likely to have a low-birth-weight infant. And while infant deaths were rare in general, common-law couples were 23 percent more likely than married ones to have a baby die in the first year of life.

The disparities persisted when the researchers accounted for factors such as the mother’s age and education, and whether she’d given birth before.

However, the findings do not necessarily point to a “therapeutic” effect of legal marriage, Kramer and his colleagues write. Women in common-law unions, they speculate, may face greater stress during pregnancy due to the relative lack of stability in their relationship-or they may differ from women who opt to walk down the aisle in some other way that affects their pregnancy.

SOURCE: Obstetrics & Gynecology, June 2004.