Background. The aim of this study was to examine the incidence and risk factors for anal sphincter tears (ASTs) at delivery. A national population-based study was conducted with data from the Medical Birth Register including all primiparas with singleton pregnancy, who gave birth vaginally in Sweden from 1994 to 2004 (n=365,886). Women with a third and fourth degree AST were compared with those who gave birth during the same period without incurring such tears. The incidence of third degree AST increased by >60%, from 3.4% in 1994 to 5.2% in 2004 in spontaneous births, and from 8.7 to 14.8% in instrumental deliveries during the study period. The proportion of fourth degree AST increased from 0.3 to 0.55% in spontaneous births and from 0.8 to 1.4% in instrumental-assisted deliveries during the same period. Compared with non-instrumental delivery, vacuum extraction (VE) deliveries were related to an increased risk of AST. An infant birth weight of >4,000 g was also associated with an increased risk for both third and fourth degree AST. In addition, women born in Africa and Asia had significantly higher risk for both third and fourth degree AST compared to women born in Sweden. The incidence of third and fourth degree AST increased in both spontaneous births and instrumental deliveries. Instrumental delivery and an infant birth weight >4,000 g are the main risk factors for AST. Women from Africa and Asia have pronounced risks.