Obstetric anal sphincter injury by maternal origin and length of residence: a nationwide cohort study

Objective To estimate the association between maternal origin and obstetric anal sphincter injury (OASI), and assess if associations differed by length of residence. Design Population-based cohort study. Setting The Medical Birth Registry of Norway. Population Primiparous women with vaginal livebirth of a singleton cephalic fetus between 2008 and 2017 (n = 188 658). Methods Multivariable logistic regression models estimated adjusted odds ratios (aORs) for OASI with 95% CI by maternal region of origin and birthplace. We stratified models on length of residence and paternal birthplace. Main outcome measures OASI. Results Overall, 6373 cases of OASI were identified (3.4% of total cohort). Women from South Asia were most likely to experience OASI (6.2%; aOR 2.24, 95% CI 1.87-2.69), followed by those from Southeast Asia, East Asia & the Pacific (5.7%; 1.59, 1.37-1.83) and Sub-Saharan Africa (5.2%; 1.85, 1.55-2.20), compared with women originating from Norway. Among women born in the same region, those with short length of residence in Norway (0-4 years), showed the highest odds of OASI. Migrant women across most regions of origin had the lowest risk of OASI if they had a Norwegian partner. Conclusions Primiparous women from Asian regions and Sub-Saharan Africa had up to two-fold risk of OASI, compared with women originating from Norway. Migrants with short residence and those with a foreign-born partner had higher risk of OASI, implying that some of the risk differential is due to sociocultural factors. Some migrants, especially new arrivals, may benefit from special attention during labour to reduce morbidity and achieve equitable outcomes. Tweetable abstract Anal sphincter injury during birth is more common among Asian and Sub-Saharan migrants and particularly among recent arrivals.