METHODS
The RCGP Oral Contraception Study started in May 1968 when 1400 general practitioners throughout the United Kingdom recruited approximately 23 000 women who were using oral contraceptives and a similar number of women who had never used this method
of birth control.1 All the women were married or living as married, most were white, and their mean age at recruitment was (SD 6.6) years. Information collected
at baseline included parity, smoking habits,social class (based on husband’s occupation), and relevant medical history. The general practitioners subsequently
supplied, at six monthly intervals, information about any hormonal preparations prescribed, any pregnancies and their outcome, all new episodes of