Differences in birthweight by maternal and paternal nativity status in Canada

Parental nativity, as well as duration of residence of foreign-born parents in the host country, has been shown to be associated with size at birth. However, most studies have focused on maternal nativity status only and have not accounted for important characteristics of both parents.

To explore whether maternal and paternal nativity and length of residence (LOR) are independently associated with birthweight for gestational age in a representative sample of infants in Canada.

We compared mean differences in sex- and gestational age-standardised birthweight z-score by nativity status of both parents in a nationally representative sample of 130,532 singleton infants born between May 2004 and May 2006 to mothers residing in Canada. We categorised parental nativity status into four groups (both parents Canada-born, mother only foreign-born, father only foreign-born and both parents foreign-born) and parents' LOR into three (both ≤10 years, only one parent ≤10 years and both >10 years). We estimated mean differences in birthweight z-score and their 95% confidence intervals in linear regression models adjusted for parity, parents' ages, education, ethnicity and marital status of the mother.

Compared with babies of Canada-born couples, those of two foreign-born parents had on average smaller birthweight z-score, -0.23 (95% CI -0.28, -0.25). However, after adjustment, the mean difference in z-score was -0.02 (95% CI -0.05, 0.00). Infants born to parents who had both resided in Canada for ≤10 years had a unadjusted mean difference in z-score of -0.27 (95% CI -0.29, -0.26), compared infants whose parents were both Canada-born, but the difference became negligible (-0.02, 95% CI -0.04, 0.01) after adjustment.

The birthweight differences by parental nativity or length of residence observed in our study population could be attributed to differences in the distribution of other parental characteristics that affect birthweight.