Associations between child and adolescent marriage and reproductive outcomes in Brazil, Ecuador, the United States and Canada

Background Although marriage is associated with favourable reproductive outcomes among adult women, it is not known whether the marriage advantage applies to girls (< 18 years). The contribution of girl child marriage (< 18 years) to perinatal health is understudied in the Americas. Methods National singleton birth registrations were used to estimate the prevalence of girl child marriage among mothers in Brazil (2011-2018, N = 23,117,661), Ecuador (2014-2018, N = 1,519,168), the USA (2014-2018, N = 18,618,283) and Canada (2008-2018, N = 3,907,610). The joint associations between marital status and maternal age groups (< 18, 18-19 and 20-24 years) with preterm birth (< 37 weeks), small-for-gestational age (SGA < 10 percentile) and repeat birth were assessed with logistic regression. Results The proportion of births to < 18-year-old mothers was 9.9% in Ecuador, 8.9% in Brazil, 1.5% in the United States and 0.9% in Canada, and marriage prevalence among < 18-year-old mothers was 3.0%, 4.8%, 3.7% and 1.7%, respectively. In fully-adjusted models, marriage was associated with lower odds of preterm birth and SGA among 20-24-year-old mothers in the four countries. Compared to unmarried 20-24-year-old women, married and unmarried < 18-year-old girls had higher odds of preterm birth in the four countries, and slightly higher odds of SGA in Brazil and Ecuador but not in the USA and Canada. In comparisons within age groups, the odds of repeat birth among < 18-year-old married mothers exceeded that of their unmarried counterparts in Ecuador [AOR: 1.99, 95%CI: 1.82, 2.18], the USA [AOR: 2.96, 95%CI: 2.79, 3.14], and Canada [AOR: 2.17, 95%CI: 1.67, 2.82], although minimally in Brazil [AOR: 1.09, 95%CI: 1.07, 1.11]. Conclusions The prevalence of births to < 18-year-old mothers varies considerably in the Americas. Girl child marriage was differentially associated with perinatal health indicators across countries, suggesting context-specific mechanisms.