"Missed pill" conception: fact or fiction?

Abstract

A study of 19 healthy nulliparous women 20-30 years who were using the low dose combined oral contraceptive (OC) demonstrated ovarian folliculogenesis during the 7-day withdrawal in 28 day cycles. This makes the genuine occurrence of conceptions seem less mythical. All of the women had normal menstrual histories before OC use. They had been on treatment for at least 3 months and had regular monthly withdrawal bleeds. Ultrasonographic examination of the ovaries was performed on the last day of 1 pack (day 21) on the last pill day (day 28) and on the 7th day of the next pack (day 7). An Elscint Dynex sector scanner with a 3.5 MHz probe was used which is capable of measuring down to 1.5 mm. All subjects showed some ovarian follicular activity by the last day of the pill-free phase and half developed multiple follicles with a dominant follicular diameter of 7 mm or greater the diameter seen on day 7 of the normal cycle. Continuation of the gonadotrophin stimulation of these dominant follicles would have led to ovulation. By the 7th day of the new pack the follicular appearance had returned to the dormant condition shown by the 1st scan in all but 1 subject and she showed reduction in follicular diameter. No pills were missed by any subject during the study cycles. In 2 subjects small cysts were detected at the 1st scan and at the 2nd and 3rd examinations unaffected by the development and regression of the other follicles. 1 subject inadvertently missed a on day 5 of a cycle after completing the study. She was examined 24 hours later and a 10 mm follicle was seen. She was unable to attend for further examination. With the current 21 day pack it is strongly recommended that alternative contraception be used when a is missed during the first 7 days of a pack even if the woman has been using OC for some time. This is most likely not necessary when a is missed later in the course. Women using OCs should also be warned of the danger of delaying restarting treatment after the 7 free days.