LAUSR

Purpose The present study aimed to assess the risk of obstetric anal sphincter injuries (OASIS) of a subsequent delivery after the previous OASIS in countries with low (Finland) and high rates (Norway and Sweden) of OASIS. Methods This population-based case–control study included women who experienced OASIS 1997–2002. 26,598 women with OASIS were included from countries with low (Finland) and high (Norway and Sweden) OASIS incidences. Each case was matched with one background-adjusted control without OASIS. A follow-up data, including all subsequent deliveries between 1998 and 2011 were then collected. Statistics significances were calculated using chi-square test, test for relative proportions and Students t test, where appropriate. Results OASIS in the first birth was associated with increased recurrences in subsequent births, 6.9% vs. 1.7% in Norway ( p  < 0.001); 4.5% vs. 0.7 ( p  < 0.001) in Sweden; and 2.1% vs. 0.8% in Finland ( p  = 0.038). In Norway, more than two deliveries occurred in 4.8% of cases and 6.2% of controls ( p  = 0.001), 4.2% vs. 5.1% in Sweden ( p  < 0.001), and 5.7% vs. 6.3% in Finland ( p  = 0.572). For women with OASIS in a previous delivery, the rates of cesarean deliveries in subsequent pregnancies were 16.4% (7.9% for controls) in Norway, and 16.3% (6.0% for controls) in Sweden, and 50.2% (14.2% for controls) in Finland. In all countries, the differences between cases and controls were significant ( p  < 0.001). Conclusion Next deliveries after OASIS are associated with increased frequency of new OASIS, more cesarean deliveries, and less subsequent deliveries in the high-risk population than women without previous OASIS.