Preterm and postterm birth in immigrant- and Swedish-born parents: a population register-based study.

Ethnic minorities/immigrant groups tend to have increased risk for preterm birth. Less is known about this risk in diverse immigrant groups, couples of mixed ethnic-origin and in relation to duration of residence. Data from the Swedish Medical Birth Register on 1,028,303 mothers who gave birth to 1,766,026 singleton live born infants (1982-2002), was linked to the Education and Total Population Registers. Immigrant parents were identified by country of birth. Risk of early preterm, late preterm and postterm birth was analyzed using multinomial logistic regression. Polish, Yugoslavian, Iranian, South Asian, East Asian and Sub-Saharan African parents, Swedish mothers who had children with non-Swedish fathers, and parents from two different immigrant groups had higher risk of early preterm birth [adjusted relative risk (RR) (95% CI) 1.76 (1.24-2.50), 1.57 (1.31-1.87), 1.67 (1.30-2.14), 1.52 (1.07-2.16), 1.51 (1.08-2.10), 2.03 (1.32-3.12), 1.56 (1.45-1.67), and 1.55 (1.35-1.77) respectively] compared to Swedish-born parents. South Asian, Sub-Saharan African, and East Asian immigrants had a higher risk of late preterm birth compared to Swedish-born parents. North African and Middle Eastern, Somali, and Ethiopian/Eritrean groups had increased risk of postterm birth [adjusted RR 1.31 (1.16-1.47), 2.57 (2.31-2.86), 1.85 (1.67-2.04) respectively]. Adjustment for covariates did not substantially change associations. Immigrant mothers resident <3 years had higher risk for early preterm and postterm birth compared to residents >10 years [adjusted RR 1.46 (1.24-1.71) and 1.16 (1.11-1.23) respectively]. In addition to higher risk of preterm birth in select immigrant groups, some immigrant groups are also at higher risk of postterm birth. Shorter duration of residence is associated with higher risk of non-term deliveries.