Here’s a look at that question from a pro-choice website: National Abortion Federation: Women Who Have Abortions:

MYTH: Women are using abortion as a method of birth control.

In fact, half of all women getting abortions report that contraception was used during the month they became pregnant.1 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%).5 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.

MYTH: Women have abortions for selfish or frivolous reasons.

The decision to have an abortion is rarely simple. Most women base their decision on several factors, the most common being lack of money and/or unreadiness to start or expand their families due to existing responsibilities. Many feel that the most responsible course of action is to wait until their situation is more suited to childrearing; 66% plan to have children when they are older, financially able to provide necessities for them, and/or in a supportive relationship with a partner so their children will have two parents.8 Others wanted to get pregnant but developed serious medical problems, learned that the fetus had severe abnormalities, or experienced some other personal crisis. About 13,000 women each year have abortions because they have become pregnant as a result of rape or incest.