Differences in birth weight by maternal and paternal nativity in Canada

BACKGROUND: Birth weight of a baby is a strong predictor of infant and neonatal survival. Several studies in various countries have compared perinatal outcomes by parental nativity. However, most are focused on maternal nativity, neglecting the father's. OBJECTIVE: Primary objective is to determine whether infant birth weight differs by maternal and paternal nativity status. Secondary objective is to examine whether there is any relation between the duration of residency in Canada of each parent and the birth weight of the offspring. METHODS: This is an observational study using 130,532 singleton, live births between May 2004 and May 2006 to mothers residing in Canada based on the 2006 Census-Birth cohort. The outcome was sex- and gestational age-standardized birth weight z-score. Parental nativity status was classified into four groups: i) both parents Canada-born, ii) mother Canada-born and father foreign-born, iii) mother foreign-born and father Canada-born, and iv) both parents foreign-born. Parents' length of residence (LOR) was classified into three groups: i) both ≤5 years, ii) either parent ≤5 years, and iii) both >5 years. We used linear regression to estimate the mean differences in birth weight z-scores and the 95% confidence interval (CI) by parental nativity status/LOR and the differences after adjusting for parity, marital status of the mother, and both parents' age, education and ethnicity. RESULTS: Crude results for parental nativity status showed that, compared with babies of two Canada-born parents, those with two foreign-born parents had a smaller adjusted birth weight for gestational age z-score [-0.266 (95% CI: -0.280, -0.251) standard deviations (SD) points]. For infants with one parent was foreign-born, the difference was -0.071 (95% CI: -0.097, -0.044) SD points for those with a foreign-born mother and -0.058 (95% CI: -0.084, -0.033) SD points for the father. When adjusted for parity, marital status of the mother, maternal and paternal age, ethnicity and education, the mean z-score difference between babies of two Canada-born parents and those with two foreign-born parents became negligible [-0.024 (95% CI: -0.049, 0.001) SD points]. When only one parent was foreign-born, the difference was -0.025 (95% CI: -0.055, 0.004) SD points for the mother and 0.003 (95% CI: - 0.025, 0.032) SD points for the father. Crude results for parents' LOR showed that, compared with babies born to Canada-born couples, those of both parents with LOR ≤5 years had on average 0.273 (95% CI: -0.296, -0.250) SD points smaller z-score. The corresponding figures were 0.130 (95% CI:-0.146, -0.113) and -0.247 (95% CI;-0.268, -0.225) for those of both parents with LOR >5 years and for those with either one parent with LOR of ≤5 years, respectively. Consistent with the primary results by parental nativity, adjustment for all covariates reduced the mean z-score differences by parents' LOR. Compared with babies of Canada-born couples, those of both parents with LOR ≤5 years had a z-score difference of -0.031 (95% CI:-0.061, -0.002) and -0.019 (95% CI:-0.040, 0.002) if parents' LOR was >5 years. If either parent's LOR was ≤5 years, the adjusted difference was -0.006 (95% CI: -0.032, 0.020) SD points. CONCLUSIONS: Our study did not find any association between parental nativity status and birth weight for gestational age and sex after adjusting for other covariates, parental ethnicity in particular. On the other hand, when adjusted for all covariates, we did observe that parents' LOR was positively associated with birth weight of the baby. Both parent's with LOR in Canada for ≤5 years, on average, had smaller babies compared with those of Canada-born parents, but no difference was found with babies born to either or both parents with LOR >5 years.

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