Kirsten Powers has a piece in the Daily Beast arguing that birth control doesn't prevent abortions. Her case is largely based on a blatant factual error. She writes:
In the U.S., the story isn’t much different. A January 2011 fact sheet by the pro-abortion rights Guttmacher Institute listed all the reasons that women who have had an abortion give for their unexpected pregnancy, and not one of them is lack of access to contraception. In fact, 54 percent of women who had abortions had used a contraceptive method, if incorrectly, in the month they got pregnant. For the 46 percent who had not used contraception, 33 percent had perceived themselves to be at low risk for pregnancy; 32 percent had had concerns about contraceptive methods; 26 percent had had unexpected sex, and 1 percent had been forced to have sex. Not one fraction of 1 percent said they got pregnant because they lacked access to contraception. Some described having unexpected sex, but all that can be said about them is that they are irresponsible, not that they felt they lacked access to contraception. [Emphasis added.]
Actually, the original study found that 12% of women who weren't using birth control when they got pregnant cited lack of access as a reason why not.
Powers' logic is as faulty as her facts. Her main evidence that birth control doesn't prevent abortions is a study of women getting abortions. If you only look at women seeking abortions, you're only going to see cases in which contraception failed, or wasn't used.
If you want to measure the power of prevention, you have to look at the millions of sexually active people who use birth control and don't get pregnant.
Publicly-funded family planning programs serve 24% of all women using reversible birth control. Publicly-funded family planning centers served 7,198,210 clients in 2006. These services prevented an estimated 1,482,800 unplanned pregnancies, and 619,000 abortions.
Powers doesn't stop to consider why comparatively few women seeking abortions mentioned lack of access. Maybe because Planned Parenthood and its sister organizations are doing a good job providing contraception to those who want it, regardless of their ability to pay. The situation will be very different if the Republicans succeed in their bid to eliminate the Title X family planning program, which covers birth control for millions of low-income Americans.