Immigrants' duration of residence and adverse birth outcomes: a population-based study

Objective This study aimed to examine preterm and small-for-gestational-age (SGA) births among immigrants, by duration of residence, and to compare them with the Canadian-born population. Design Population-based cross-sectional study with retrospective assessment of immigration. Setting Metropolitan areas of Ontario, Canada. Population A total of 83 233 singleton newborns born to immigrant mothers and 314 237 newborns born to non-immigrant mothers. Methods We linked a database of immigrants acquiring permanent residence in Ontario, Canada, in the period 1985-2000 with mother-infant hospital records (2002-2007). Duration of residence was measured as completed years from arrival to Canada to delivery/birth. Logistic regression models were used to estimate the effects of duration of residence with adjusted odds ratios and 95% confidence intervals. In analyses restricted to immigrants only, hierarchical models were used to account for the clustering of births into maternal countries of birth. Main outcome measures Preterm birth (PTB) and SGA birth. Results Recent immigrants (< 5 years) had a lower risk of PTB (4.7%) than non-immigrants (6.2%), but those with >= 15 years of stay were at higher risk (7.4%). Among immigrants, a 5-year increase in Canadian residence was associated with an increase in PTB (AOR 1.14, 95% CI 1.10-1.19), but not in SGA birth (AOR 0.99, 95% CI 0.96-1.02). Conclusions Time since migration was associated with increases in the risk of PTB, but was not associated with an increase in SGA births. Ignoring duration of residence may mask important disparities in preterm delivery between immigrants and non-immigrants, and between immigrant subgroups categorised by their duration of residence.