This may be due to the mistaken belief that women with anorexia can't get pregnant because they have irregular menstrual periods or no periods at all, said study lead author Cynthia M. Bulik, director of the eating disorders programme at the University of North Carolina (UNC).
"Anorexia is not a good contraceptive. Just because you're not menstruating, or because you're menstruating irregularly, doesn't mean you're not at risk for becoming pregnant," she said.
Bulik and colleagues analysed data on 62,060 women included in the Norwegian Mother and Child Cohort Study and identified 62 who reported having anorexia nervosa. Unplanned pregnancies were reported by 50% of women with anorexia and 18.9% of other women.
The study also found that 24.2% of women with anorexia said they had a prior abortion, compared with 14.6% of other women.
The findings are published in the journal Obstetrics & Gynecology.
Physicians and other health care providers "need to make sure that they have the conversation about sexuality and contraception as clearly with patients with anorexia as they do with all other girls and women," Bulik said.