They may inhibit or delay ovulation to prevent fertilization. No contraceptive was used when intercourse took place. A male condom slipped, broke, or leaked. A women's diaphragm or cervical cap was inserted incorrectly, dislodged during intercourse, was removed too early, or was found to be torn. Because combined pills act by suppressing ovulation, the risk of conception occurs some days after the missed pills when follicular development, freed from the inhibition by the oral contraceptive, is sufficient to allow ovulation.
Ovulation is unlikely if only a few pills have been missed during a cycle.
Nevertheless, treatment is reasonable if the woman is worried or wishes to avoid even a small risk of pregnancy. A woman has missed one or more progestin-only pills. In contrast to combined pills, ovulation is not consistently suppressed by progestin-only pills, and the risk of pregnancy after missing progestinonly pills is greater and more immediate than is the case with missed combined pills. A female condom was inserted or removed incorrectly leading to spillage of semen, or the penis was inserted mistakenly between the female condom and the vaginal wall resulting in intravaginal ejaculation. An IUD was partially or totally expelled.