The joint influence of marital status, interpregnancy interval, and neighborhood on small for gestational age birth: a retrospective cohort study

Abstract

Background: Interpregnancy interval (IPI), marital status, and neighborhood are independently associated with birth outcomes. The joint contribution of these exposures has not been evaluated. We tested for effect modification between IPI and marriage, controlling for neighborhood.

Methods: We analyzed a cohort of 98,330 live births in Montréal, Canada from 1997-2001 to assess IPI and marital status in relation to small for gestational age (SGA) birth. Births were categorized as subsequent-born with short (<12 months), intermediate (12-35 months), or long (36+ months) IPI, or as firstborn. The data had a 2-level hierarchical structure, with births nested in 49 neighborhoods. We used multilevel logistic regression to obtain adjusted effect estimates.

Results: Marital status modified the association between IPI and SGA birth. Being unmarried relative to married was associated with SGA birth for all IPI categories, particularly for subsequent births with short (odds ratio [OR] 1.60, 95% confidence interval [CI] 1.31-1.95) and intermediate (OR 1.48, 95% CI 1.26-1.74) IPIs. Subsequent births had a lower likelihood of SGA birth than firstborns. Intermediate IPIs were more protective for married (OR 0.50, 95% CI 0.47-0.54) than unmarried mothers (OR 0.65, 95% CI 0.56-0.76).

Conclusion: Being unmarried increases the likelihood of SGA birth as the IPI shortens, and the protective effect of intermediate IPIs is reduced in unmarried mothers. Marital status should be considered in recommending particular IPIs as an intervention to improve birth outcomes.

Figure 1

Odds of SGA birth for unmarried relative to married mothers, according to interpregnancy interval and maternal place of birth, singleton births, Montréal, Canada, 1997 to 2001. Results are from multi-level logistic regression testing an interaction term between marital status and interpregnancy interval, adjusting for maternal age, education, country of birth, year of birth, interaction between marital status and country of birth, neighborhood perceived security, and neighborhood proportion foreign-born population. Odds ratios are for unmarried relative to married mothers.

Figure 2

Odds of SGA birth for interpregnancy interval relative to firstborns, stratified by marital status, singleton births, Montréal, Canada, 1997 to 2001. Results are from multi-level logistic regression testing an interaction term between marital status and interpregnancy interval, adjusting for maternal age, education, country of birth, year of birth, interaction between marital status and country of birth, neighborhood perceived security, and neighborhood proportion foreign-born population. Odds ratios are for interpregnancy interval relative firstborns.