Background: People who immigrate to Western nations may experience fewer chronic health problems than original residents of those countries, which raises concerns about long-term environmental or lifestyle factors in those countries. We tested whether the "healthy immigrant effect" extends to the risk of placental dysfunction during the short interval of pregnancy.
Methods: We conducted a population-based retrospective cohort study of data for 796 105 women who had a first documented obstetric delivery in Ontario between 1995 and 2005. Recency of immigration was determined for each woman as the time from her enrolment in universal health insurance to her date of delivery, classified as less than 3 months, 3-5 months, 6-11 months, 12-23 months, 24-35 months, 36-47 months, 48-59 months and 5 years or more (the referent). The primary composite outcome was maternal placental syndrome (defined as a diagnosis of pre-eclampsia or eclampsia, placental abruption or placental infarction).
Results: The mean age of the women was 28.8 years. Maternal placental syndrome occurred in 45 216 women (5.7%). The risk of this outcome was lowest among the women who had immigrated less than 3 months before delivery (3.8%) and highest among those living in Ontario at least 5 years (6.0%), for a crude odds ratio (OR) of 0.62 (95% confidence interval [CI] 0.54-0.71). After adjustment for maternal age, income status, pre-existing hypertension, diabetes mellitus, multiple gestation and receipt of prenatal ultrasonography, the risk of maternal placental syndrome was correlated with the number of months since immigration in a gradient manner (OR, 95% CI): less than 3 months (0.53, 0.47-0.61), 3-5 months (0.68, 0.61-0.76), 6-11 months (0.67, 0.63-0.71), 12-23 months (0.69, 0.66-0.73), 24-35 months (0.75, 0.70-0.79), 36-47 months (0.75, 0.70-0.80) and 48-59 months (0.82, 0.77-0.87).
Interpretation: There was a progressively lower risk of maternal placental syndromes associated with recency of immigration. The "healthy immigrant effect" may extend to common placental disorders, diminishes with the duration of residency and underscores the importance of nongenetic determinants of maternal health accrued over a brief period.