Pill method failures in women seeking abortion: fourteen years experience.

Aims: To document predisposing factors in 769 women who had inadvertently conceived while taking oral contraceptive pills reliably.

Methods: Over a 14-year period, 1981-1995, two questionnaires were administered, one for women using the combined oral contraceptive pill (COC) and one for women using the progestogen only pill (POP).

Results: More than one predisposing factor was usually present, the most common being diarrhoea and/or vomiting in 39.0%. Antibiotics had been prescribed in 20.5%. COC failure was associated with a high incidence of menstrual disturbance (37.1%). A history of a previous pill failure was obtained in 27.8%. Smoking was a risk factor for women on the COC. Only 6.0% became pregnant during the first month of pill use. As expected, POP users experienced a higher number of pill failures in proportion to the market share, but excess weight and time of taking did not appear to be risk factors. Most COCs failed according to the numbers expected from the market share.

Conclusions: Many pill failures could be averted by better information about known or suspected risk factors, especially gastrointestinal upset and interacting medications. Pill failure is one more reason to discourage smoking in COC users.