Only few studies exist regarding the risk of a teratogenic effect of tramadol when used in early pregnancy. Using the Swedish Medical Birth Register, women (deliveries in 1997-2013) who had reported the use of tramadol in early pregnancy were identified. Maternal characteristics and concomitant drug use were analyzed. Among 1,682,846 women (1,797,678 infants), 1751 (1776 infants) had used tramadol, 96 of the infants had a congenital malformation and 70 of them were relatively severe. The adjusted odds ratio for a relatively severe malformation was 1.33 (95% CI 1.05-1.70). The odds ratios for cardiovascular defects (1.56, 95% CI 1.04-2.29) and for pes equinovarus (3.63, 95% CI 1.61-6.89) were significantly increased. The study suggests a teratogenic effect of tramadol but the risk increase is moderate. (C) 2015 Elsevier Inc. All rights reserved.