This article is from the Big Folks Health FAQ, by firstname.lastname@example.org (Sharon Curtis) with numerous contributions by others.
Body fat can have an effect on menstruation, in particular because
estrogen is produced by fat cells (as well as from the ovaries). Very
thin women (e.g. anorexics or very fit athletes) tend to stop ovulating
because their production of estrogen is low due to the lack of body fat.
In contrast, fat women have plenty of estrogen. In very obese women,
this can play havoc with the menstrual cycle, either lack of
menstruation, or irregular menstruation. For regularizing menstruation,
where pregnancy is not required, oral contraceptives may prove useful.
If pregnancy is required, there are drugs or hormones that can be
taken to help normal egg production.