Contraceptives are not exactly fool-proof. There is still a chance that contraceptives will fail and women will still get pregnant. Some women get pregnant because they have forgotten to take the pill. The birth control pills that are available in the market right now are being used in order to regulate the hormones of women.
If taking one pill is missed, this can change the hormonal levels of the body and cause the body to start ovulating. Women are fertile during the ovulation stage. If they happen to have sex during this time, they have a higher chance of getting pregnant. Some women who try to terminate the pregnancy may get an abnormal pregnancy instead.