There aren't any set formulas for determining when a nursing mother will resume her periods.
Some women produce enough oestrogen to begin menstruating again even before they've weaned their babies, sometimes as early as 6 weeks - 3 months postnatal. But others, particularly those who have breastfed for a long time, nursed exclusively or had irregular menstrual periods before pregnancy, will have a break from menstruation until 7 months after they've weaned. Be sure, however, that you're eating enough and haven't been losing weight too quickly; as strenuous dieting especially when combined with strenuous exercising can temporarily hold off the return of the menstrual cycle.
Keep in mind that just because you're not getting your period doesn't mean you can't get pregnant again!
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