In fact, half of all women getting abortions report that contraception was used during the month they became pregnant. Some of these couples had used the method improperly; some had forgotten or neglected to use it on the particular occasion they conceived; and some had used a contraceptive that failed.
No contraceptive method prevents pregnancy 100% of the time. If abortion were used as a primary method of birth control, a typical woman would have at least two or three pregnancies per year — 30 or more during her lifetime.
In fact, most women who have abortions have had no previous abortions (52%) or only one previous abortion (26%). Considering that most women are fertile for over 30 years, and that birth control is not perfect, the likelihood of having one or two unintended pregnancies is very high.