Back to Dana Stevens, and "the 77 percent of Americans who support abortion rights—and the 40 percent or more of American women who have exercised that right." Ramesh says most of what needs to be said about the first statistic, which may be technically "true," but only if you count as "pro-choice" voters who support legal abortion in cases of rape, incest, fetal deformity, and so forth. (That is, in a vanishingly small percentage of all abortions.) The numbers on abortion are almost infinitely malleable, depending on how you ask the question, but there seems to be a consistent constituency of around forty percent for the current abortion regime, around twenty percent for the strict pro-life positions, and around forty percent for further restrictions of varying degrees. Pro-lifers like to say that seventy percent of Americans oppose ninety percent of abortions (or variations on that theme), which is a little bit of a stretch, but at least as close to the truth as what Stevens is claiming.
Stevens' second statistic - the percentage of American women who have had abortions - is likewise dubious, though it may not be all that far off. I haven't found a really rigorous analysis of this question, but the Guttmacher Institute says "at current rates more than one-third will have had an abortion by age 45," while the National Abortion Federation (presumably drawing on the same data) says "35% of all women of reproductive age in America today will have had an abortion by the time they reach the age of 45." So not quite forty percent, but within hailing distance. I can't find the underlying data that Guttmacher and the NAF are using, so I did some of back-of-the-envelope math using this Guttmacher figure:
Given that almost half of all abortions are repeat abortions, my calculations suggest that if the 2001 rate held for the following twenty-nine years, a girl who was fifteen in that year would have roughly a 29 percent chance of having at least one abortion over her reproductive lifetime. That's lower than the Guttmacher estimate, but then again the '01 rate was the lowest in a generation; if the far higher 1981 rate held for a generation, my back-of-the-envelope math suggests that forty percent of women would have at least one abortion over that span. So maybe that's where Stevens' number comes from, and the 35 percent number that Guttmacher cites averages out the last couple decades. But if there's a more detailed analysis out there I'd love to see it.