Adverse birth outcomes in relation to maternal marital and cohabitation status in Canada

An increasing percentage of children are born to couples who cohabit but are not legally married. Using data from a nationally representative Canadian sample, we estimated associations of maternal marital and cohabitation status with stillbirth, infant mortality, preterm birth (PTB), and small- and large-for-gestational-age (SGA and LGA) birth.

The 2006 Canadian Birth-Census Cohort was created by linking birth registration data with the 2006 long-form census. We used log-binomial regression to estimate risk ratios (RRs) for adverse birth outcomes associated with being single or living with a common-law partner. Analyses were adjusted for maternal age and education.

Data were analyzed for 130,931 singleton births. Adjusted RRs (95% confidence intervals) for single mothers compared with married mothers were 1.92 (1.51-2.42) for stillbirth, 2.08 (1.55-2.81) for infant mortality, 1.36 (1.27-1.46) for PTB, 1.31 (1.22-1.39) for SGA birth, and 0.95 (0.90-1.01) for LGA birth. Adjusted RRs for cohabiting mothers compared with married mothers were 0.93 (0.74-1.16) for stillbirth, 1.05 (0.81-1.35) for infant mortality, 1.09 (1.03-1.15) for PTB, 1.05 (0.99-1.10) for SGA birth, and 0.96 (0.92-1.00) for LGA birth.

In a nationally representative Canadian birth cohort, cohabiting and legally married women experienced similar birth outcomes, but most outcomes for single women were substantially worse.