What were the results of the cohort study?
Danish women aged 15-49 with no history of cardiovascular or malignant disease were recruited to this study. It linked the data of all women in a National Registry of Patients collected from all Danish hospitals since 1977 with data on their oral contraceptive use from a national registry of prescriptions. A total of 10.4million ‘woman years’ of data were recorded. A ‘woman year’ is a statistical concept of one woman’s data collected for a year. In this concept, five women followed for one year contribute the same amount of data to this study as one woman followed for five years. The analysis included 3.4million woman years of current use of oral contraceptives, 2.3million woman years of former use, 4.8million woman years of never use, giving a total of about 10.4million woman years of observation. A total of 4,213 first-time venous thrombotic events were recorded during the study period. Of these, 2,045 were current users of hormonal contraception. The venous thrombotic events include
