Editor,—A recent clinical alert (Gut1999;44:311–12) commented on a study of risk factors for relapse in Crohn's disease.1 The author concluded that, unlike the established association with smoking, the link with oral contraceptive use is still controversial. To contribute to this discussion, I have investigated temporal trends in age and sex specific disease incidence and correlated them with the chronology of oral contraceptive use.

The birth control pill was first introduced in 1960 and soon became widely accepted; 10 million American women were taking it by 1973.2 Concerns about side effects prompted further research, and by the mid-1970s most women taking were taking oral contraceptive pills containing 50 μg or less of oestrogen—a considerable decrease from the …