The joint influence of marital status, interpregnancy interval, and neighborhood on small for gestational age birth: a retrospective cohort study
1 Unité Études et analyses de l'état de santé de la population, Institut national de santé publique du Québec, Montréal, Québec, Canada
2 Département de médecine sociale et préventive, Université de Montréal, Montréal, Québec, Canada
3 School of Health Sciences, University of South Australia, Adelaide, Australia
4 Department of Epidemiology and Biostatistics, McGill University, Montréal, Québec, Canada
5 Hospital Sainte Justine Research Centre, Department of Obstetrics and Gynaecology, Université de Montréal, Montréal, Québec, Canada
BMC Pregnancy and Childbirth 2008, 8:7 doi:10.1186/1471-2393-8-7Published: 28 February 2008
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.
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.
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).
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.