OBJECTIVE: To assess the risks and trends of adverse pregnancy outcomes among mothers in common-law unions versus traditional marriage relationships. METHODS: We conducted a birth cohort-based study of all 720,586 births registered in Quebec for the years 1990 to 1997. RESULTS: The proportion of births to common-law mothers more than doubled from 20 in 1990 to 44 in 1997. Preterm birth, low birth weight, small for gestational age, and neonatal and postneonatal mortality rates increased progressively from mothers legally married, to common-law unions, to lone mothers with father information, to lone mothers without father information on birth registrations. Adjusted odd ratios with 95 confidence intervals (CIs) for common-law versus legally married mothers were 1.14 (95 CI 1.11, 1.17) for preterm birth, 1.21 (95 CI 1.18, 1.25) for low birth weight, 1.18 (95 CI 1.16, 1.20) for small for gestational age, 1.07 (95 0.97, 1.19) for neonatal death, and 1.23 (95 CI 1.04, 1.44) for postneonatal death after controlled for observed individual- and community-level characteristics. The crude and adjusted odds ratios were virtually unchanged over time. CONCLUSION: Pregnancy outcomes are worse among mothers in common-law unions versus traditional marriage relationships but better than among mothers living alone. Modest disparities in pregnancy outcomes in common-law versus traditional marriage relationships have persisted despite the striking rise in common-law unions. LEVEL OF EVIDENCE: II-2
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