Common-law unions tied to infant death and other negative birth outcomes.

Despite the growing prevalence and acceptability of common-law marriages in Quebec, women in this type of union continue to be at greater risk for negative birth outcomes than those in traditional marriages. (1) According to a study of births registered in the province in 1990-1997, infants born to women in cohabiting relationships are significantly more likely than those born to legally married women to be preterm, small for gestational age or low-birth-weight, or to die within 28-364 days. The odds of these outcomes are even higher among infants born to single women, particularly if the father's age and birthplace are not recorded on the birth registration form.

Data were drawn from birth registration files and linked vital records for 714,748 live births, including information on mothers' demographic characteristics and relationship status, and fathers' age and birthplace. Available data permitted the researchers to assess the prevalence of a number of birth outcomes, including preterm delivery (before 37 weeks' gestation), small size for gestational age (less than the 10th percentile, based on national standards for 1994-1996), low birth weight (less than 2,500 g), neonatal death (0-27 days) and postneonatal death (28-364 days). The researchers used chi-square tests to identify significant differences among infants born to four groups of women: those who were legally married, those who were in cohabiting relationships, those who were single and provided any paternal information, and those who were single and provided no paternal information. Multilevel logistic regressions were constructed to examine associations between mothers' relationship status and various birth outcomes.

Throughout the study period, 53% of births were to women who were legally married, 35% were to women in common-law unions, 8% were to single mothers with paternal information and 4% were to single mothers with no paternal information. Between 1990 and 1997, the proportion of births to women in cohabiting relationships increased from 20% to 44%, whereas the proportion of births to legally married women decreased from 62% to 46%.

Overall, 36-59% of births were to women who had no other children, 1-22% were to teenagers and 10-43% were to women who had had less than 11 years of schooling. Proportions increased significantly across study groups, as mothers' relationship status went from legally married to cohabiting to single with paternal information to single with no paternal information. The proportion of infants born to native French speakers was significantly higher among children of women in common-law unions than among those of women in other groups (93% vs. 71-83%), as was the proportion of infants born to women who lived in communities of fewer than 10,000 people (25% vs. 19-20%).

Among the four study groups, 6-11% of infants were preterm, 9-17% were small for gestational age and 5-10% were low-birth-weight; the rate of neonatal death was 3-6 per 1,000 live births, and the rate of postneonatal death was 1-3 per 1,000 neonatal survivors. The likelihood of these outcomes increased significantly as mothers' relationship to fathers became less formal.

Results of the regression analysis, which controlled for mothers' demographic characteristics and infants' gender, showed significant associations between marital status and a number of negative birth outcomes: Infants born to women in cohabiting partnerships were more likely than those with mothers in traditional marriages to be preterm (odds ratio, 1.1), small for gestational age (1.2) or low-birth-weight (1.2), or to die in the postneonatal period (1.2). These findings remained consistent when the researchers compared results for 1990-1993 with those for 1994-1997.

Relative to infants whose mothers were legally married, infants born to single mothers with paternal information had significantly elevated odds of preterm delivery (odds ratio, 1.3), small size for gestational age (1.3), low birth weight (1.5), neonatal death (1.3) or postneonatal death (1.4). Infants born to single mothers with no paternal information were significantly more likely than those born to legally married women to be preterm (1.4), small for gestational age (1.5) or low-birth-weight (1.6), or to die during the neonatal or postneonatal period (1.3 and 1.7).

The researchers acknowledge that their ability to explain the study findings is limited by the absence of information on a number of clinical and lifestyle factors, such as smoking and income level. They note that more research is needed to investigate the "causal mechanisms underlying the observed persistent disparities in pregnancy outcomes among common-law versus legally married mothers," especially in light of the increasing popularity of common-law unions. These disparities, they suggest, "may well be related to differences among women who enter varying personal and cohabitation relationships, or to the quality of the relationships themselves, rather than to a beneficial effect of the legal act of marriage."

REFERENCE

(1.) Luo ZC, Wilkins R and Kramer MS, Disparities in pregnancy outcomes according to marital and cohabitation status, Obstetrics & Gynecology, 2004, 103(6):1300-1307.