Birth outcomes of foreign-born, native-born, and mixed couples in Sweden.

Background: Many births in industrialised countries are to immigrant parents, or to one immigrant and one domestically born parent. Their newborn outcomes have not been well studied.

Methods: We conducted a study of 1,690,423 singleton infants born in Sweden between 1987 and 2008, including those of immigrants from East Asia, South Asia, Sub-Saharan Africa, and East Africa. Preterm delivery and small for gestational age (SGA) were assessed among infants of (i) immigrant parents from the same world region, (ii) an immigrant mother and a Swedish-born father, and (iii) a Swedish-born mother and an immigrant father; each compared to (iv) two Swedish-born parents. Log binomial regression analysis generated adjusted risk ratios (ARRs) and 95% confidence intervals (CIs) for preterm delivery and SGA.

Results: Compared with infants of two Swedish-born parents, infants born to immigrant mothers from East Asia, South Asia, and Sub-Saharan African were at higher risk of preterm delivery (ARR ranging from 1.2 to 1.9), irrespective of whether the father was from the same world region or Swedish-born, with the only exception of East African women, who had lower risk. Infants born to two foreign-born parents had the highest risks of SGA, particularly South Asians (ARR 4.69; 95% CI 4.29, 5.12). Mixed couples exhibited intermediate risks of SGA.

Conclusions: Adverse birth outcomes differ according to a couple's ethnic composition. Having a Swedish-born partner is associated with lower risk of SGA among immigrant mothers and fathers, and with lower risk of preterm delivery among immigrant fathers but not mothers.