By being noninvasive, medical termination of pregnancy has increased worldwide access to abortion and improved safety of unsafe abortion. However, secondary surgical intervention is the most frequent complication to medical abortion. We aimed to identify and quantify risk factors for surgical intervention in women undergoing medically induced termination of pregnancy before 9 completed weeks of gestation. We conducted a nationwide cohort study, including all pregnancies terminated before 63 gestational days in women aged 15-49 years during the period 2005-2015. Induction regimen was 200 mg mifepristone followed 24-48 hours later by 0.8 mg vaginal misoprostol. All included pregnancies were followed up for 8 weeks from mifepristone administration. Data were retrieved from national health registers. Multiple logistic regression provided adjusted odds ratios of surgical intervention with 95% confidence intervals. The discriminative ability of the risk factors in identifying surgical intervention was assessed by cross-validated area under the receiver operating characteristic curve. Of 86,437 early medical abortions, 5320 (6.2%) underwent a surgical intervention within 8 weeks after induction. The proportion of surgical interventions increased from 3.5% in the 5th to 6th gestational week to 10.3% in week 9, odds ratio, 3.2 (95% confidence interval, 2.9-3.6). Compared with women aged 15-19 years, the risk of surgical intervention increased with increasing maternal age until the age of 30-34 years, odds ratio, 1.7 (95% confidence interval, 1.5-1.9), where after the risk decreased to an odds ratio for age group 40-49 of 1.2 (95% confidence interval, 1.0-1.4). Compared with nulliparous women, a history of only vaginal deliveries with spontaneous delivery of placenta implied an odds ratio of 1.1 (95% confidence interval, 1.0-1.2), women with a history of at least 1 cesarean delivery, an odds ratio of 1.5 (95% confidence interval, 1.3-1.6), and women having experienced a manual removal of placenta after a vaginal birth, an odds ratio of 2.0 (95% confidence interval, 1.7-2.4). Previous medically induced abortion decreased the risk of surgical intervention, odds ratio 0.84 (95% confidence interval, 0.78-0.91), whereas previous early (before 56 days of gestation) surgically induced abortion implied a 53% (95% confidence interval, 1.4-1.7) increased risk of surgical intervention. Previous surgical abortion after 55 days of gestation increased the risk by 17% (95% confidence interval, 1.1-1.3). The area under the receiver operating characteristic curve of the model including all quantified risk factors was 63% (95% confidence interval, 62-64%). Gestational age, maternal age, previous deliveries, and history of medically and surgically induced abortions all had a significant influence on the risk of surgical intervention of early medical abortion. However, inclusion of all quantified risk factors still left most interventions unpredictable.