(12 periods a year) x (around 30 years of premenopausal adulthood) = about 360 chances to get pregnant. If birth control, when used properly, is about 99% effective, then that's 3-4 accidental pregnancies likely, statistically.That presumes perfect usage of a method like the pill; less effective methods such as condoms are likely to increase the chances of having multiple contraceptive failures over a lifetime. Should this make us wary of presuming that the one half of women getting abortions who've had one before are irresponsible?
UPDATE: in the comments, more statistically literate folks point out that the effectiveness rates are calculated based on a year of use and thus 99% effective birth control would yield an expected 0.3 accidental pregnancies over 30 years. Kyra at Pandagon was wrong!
