Obstetric anal sphincter injuries (OASIS) can complicate up to 6% of births and are a major contributor to preventable maternal morbidity. Asian women have a risk of third and fourth degree perineal tears up to four times greater than women of other ethnicities in the same community, but the lack of differentiation of Asian women into regional groups has limited insight into the reasons behind their increased risk.

To investigate risk of OASIS associated with country of birth.

This was a retrospective cohort study of all women with a singleton, nulliparous pregnancy who delivered vaginally by spontaneous vaginal birth or an instrumental delivery between 1 January 2009 and 31 December 2015. The demographics of women who experienced OASIS were compared with those women who had minor perineal trauma.

From January 2009 to December 2015 there were 10 750 singleton, nulliparous and natural vaginal birth (NVB), forceps or vacuum deliveries. Of these deliveries, 581 (5.4%) werehad third degree tears and 36 (0.3%) fourth degree tears. Women born in South Asia were at a much higher risk of OASIS than other groups, including women born in other Asian countries, compared to the Australian/New Zealand cohort. One in every 10 nulliparous South Asian women having a singleton vaginal or instrumental delivery will sustain an OASIS.

Our study further confirms the role of Asian ethnicity in the risk of OASIS, and is the second to confirm that South Asian women are at a dramatically increased risk.