Lydia McGrew dissects the study (pdf) upon which the much cited statistic is based:
The survey was limited to women between 15-44. Ah, well, that explains how we weren't including the elderly, but it also means that the silly "percent of all Catholic women" thing should be chucked out right from the beginning. More strikingly, as Neil pointed out to me after looking up the study, it excluded any women who were a) not sexually active, where that is defined as having had sexual intercourse in the past three months (there go all the nuns), b) postpartum, c) pregnant, or d) trying to get pregnant!
In other words, the study was specifically designed (as the prose discussion on p. 8 makes explicit, in bold print) to include only women for whom a pregnancy would be unintended and who are "at risk" of becoming pregnant. Whether or not it included women who considered themselves neither trying nor not trying to get pregnant (there are some such women in the world) is unclear. It's also unclear whether it included women who have had their reproductive organs removed because of some medical problem. Presumably the study was intended to exclude women in both of these categories, as neither would count as a woman "at risk of an unintended pregnancy."
The Dish 