Objectives. We estimated the associations of parity and offspring birthweight with the risk of shoulder dystocia, and studied whether the association of offspring birthweight differed by parity. Design. Population-based register study. Setting. The Medical Birth Registry of Norway was used to identify all deliveries between 1967 and 2006. Population. All vaginal deliveries of a singleton offspring in cephalic presentation during the period 1967–2006 (n=1 914 544). Main outcome measure. Shoulder dystocia at delivery. Results: Shoulder dystocia occurred in 0.68% (13 109/1 914 544) of all deliveries. There was a strong positive association of birthweight with risk of shoulder dystocia, and 75% (9765/13 109) of all cases occurred in deliveries of offspring weighing 4000g or more. The association of birthweight displayed similar patterns across parities, but the association was slightly stronger in parous than in primiparous women. Among first-time mothers, 0.12% (320/276 614) with offspring weighing 3000–3499g (reference) experienced shoulder dystocia, compared with 13.30% (169/1244) with offspring birthweight higher than 5000g [odds ratio (OR) 135.7, 95%CI 111.6–165.1]. The corresponding results for women with one previous delivery were 0.08% (161/201 572) and 16.45% (501/3054) (OR 246.4, 95%CI 205.4–295.5). Conclusions. High offspring birthweight is the major risk factor for shoulder dystocia, constituting most cases. The positive association of birthweight with shoulder dystocia showed similar patterns across parities, but high birthweight parous women were at greater risk of shoulder dystocia compared with primiparous women.