Maternal country of origin and infant birthplace: implications for birth-weight

Does Maternal Country of Birth Matter for Understanding Offspring's Birthweight? A Multilevel Analysis of Individual Heterogeneity in Sweden

Background

Many public health and epidemiological studies have found differences between populations (e.g. maternal countries of birth) in average values of a health indicator (e.g. mean offspring birthweight). However, the approach based solely on population-level averages compromises our understanding of variability in individuals’ health around the averages. If this variability is high, the exclusive study of averages may give misleading information. This idea is relevant when investigating country of birth differences in health.

Methods and Results

To exemplify this concept, we use information from the Swedish Medical Birth Register (2002–2010) and apply multilevel regression analysis of birthweight, with babies (n = 811,329) at the first, mothers (n = 571,876) at the second, and maternal countries of birth (n = 109) at the third level. We disentangle offspring, maternal and maternal country of birth components of the total offspring heterogeneity in birthweight for babies born within the normal timespan (37–42 weeks). We found that of such birthweight variation about 50% was at the baby level, 47% at the maternal level and only 3% at the maternal countries of birth level.

Conclusion

In spite of seemingly large differences in average birthweight among maternal countries of birth (range 3290–3677g), knowledge of the maternal country of birth does not provide accurate information for ascertaining individual offspring birthweight because of the high inter-offspring heterogeneity around country averages. Our study exemplifies the need for a better understanding of individual health diversity for which group averages may provide insufficient and even misleading information. The analytical approach we outline is therefore relevant to investigations of country of birth (and ethnic) differences in health in general.

Testing the "epidemiologic paradox" of birth outcomes among Asian immigrant women in Hsin-Chu County, Taiwan

BACKGROUND/PURPOSE

Taiwan saw an increase in immigration during the last decade. This retrospective study investigated whether immigrant status confers a protective effect on birth outcomes and whether this effect varies across racial/ethnic subgroups in Hsin-Chu County, Taiwan.

METHODS

A total of 30,770 singleton birth certificates from January 1, 2002 to July 31, 2007 were analyzed using ANOVA and logistic regression. Outcomes included low birth weight (LBW, < 2500 g), high birth weight (HBW, > 4000 g), preterm birth (< 37 weeks) and stillbirth. Covariates included maternal age, year of delivery, mode of delivery, medical care institution, infant sex and congenital birth defects. Five models were designed for various analyses. The reference categories included non-immigrant, non-aboriginal Taiwanese and non-Chinese-speaking immigrants.

RESULTS

Preterm rate ranged from 6.3% among mainland Chinese to 13.5% among aboriginal Taiwanese. LBW rate ranged from 4.3% among mainland Chinese to 17.3% among aboriginal Taiwanese. HBW rate ranged from 1.2% among aboriginal Taiwanese to 3.4% among mainland Chinese. Stillbirth rate ranged from 0.2% among Indonesians to 0.7% among aboriginal Taiwanese. Adjusted odds ratio (OR) was lower among mainland Chinese (preterm OR, 0.77; LBW OR, 0.62) but higher among aboriginal Taiwanese (preterm OR, 1.79; LBW OR, 2.68; stillbirth OR, 2.92). HBW rate was significantly higher (OR, 1.84) among mainland Chinese. Chinese-speaking immigrants showed significant differences in LBW (OR, 0.57) and HBW (OR, 1.62) compared with non-Chinese-speaking immigrants.

CONCLUSION

An epidemiologic paradox and heterogeneity of birth outcomes were observed among immigrants in this study. However, aboriginal Taiwanese constituted the subgroup with the highest risk. Further research is needed to identify the determinants of birth outcomes.