PURPOSE: Several studies have been conducted to assess determinants affecting the performance or accuracy of self-reports. These studies are often not focused on pregnant women, or medical records were used as a data source where it is unclear if medications have been dispensed. Therefore, our objective was to evaluate the concordance between self-reported medication data and pharmacy records among pregnant women and its determinants. METHODS: We conducted a population-based cohort study within the Generation R study, in 2637 pregnant women. The concordance between self-reported medication data and pharmacy records was calculated for different therapeutic classes using Yule's Y. We evaluated a number of variables as determinant of discordance between both sources through univariate and multivariate logistic regression analysis. RESULTS: The concordance between self-reports and pharmacy records was moderate to good for medications used for chronic conditions, such as selective serotonin reuptake inhibitors or anti-asthmatic medications (0.88 and 0.68, respectively). Medications that are used occasionally, such as antibiotics, had a lower concordance (0.51). Women with a Turkish or other non-Western background were more likely to demonstrate discordance between pharmacy records and self-reported data compared with women with a Dutch background (Turkish: odds ratio, 1.63; 95% confidence interval, 1.16-2.29; other non-Western: odds ratio, 1.33; 95% confidence interval, 1.03-1.71). CONCLUSIONS: Further research is needed to assess how the cultural or ethnic differences may affect the concordance or discordance between both medication sources. The results of this study showed that the use of multiple sources is needed to have a good estimation of the medication use during pregnancy.