The researchers explain these counter-intuitive findings like so, "The high prevalence of menstrual disturbance has contributed to the (mis)conception that women with anorexia nervosa are unlikely to conceive." In one of the funnier lines ever to be included in the sober press release for a scientific study, lead researcher Cynthia M. Bulik simply says: "Anorexia is not a good contraceptive." You see, just because a woman isn't menstruating doesn't mean she can't get pregnant. The more you know! Somehow that hugely important fact never made it into my high school sex-ed curriculum. In fact, I'm embarrassed to admit that at the age of 26 -- having written about women's issues five days a week for the past four years -- this is the first time I've heard it.
The study speculates that it's "possible that absent or irregular menstruation and the belief that menstrual irregularities reduce the risk of conception are associated with reduced adherence to contraception guidelines or instructions, increasing the risk of unplanned pregnancy" -- but that hypothesis still has to be borne out in further research. Regardless, it seems a safe bet to at least begin better educating women, anorexic and otherwise, about the risk of pregnancy even if they aren't menstruating.
http://www.salon.com/life/pregnancy/index.html?story=/mwt/broadsheet/2010/11/12/anorexia_pregnancy