Is it normal not to have periods (menstruate) when breastfeeding?
Human beings have known for centuries that breastfeeding affects fertility, and this has been borne out in recent studies. The individual variations are, however, great. In general, the more often a baby is breastfed, the younger the baby is, and the less nutrition he gets from other sources, the later the mother's periods will resume. Most breastfeeding mothers will resume their periods between 6 months and 2 years after their baby's birth. Weaning will almost certainly cause a resumption of the menstrual cycle.
A woman who wants to use this knowledge for purposes of regulating her fertility should inform herself about the "Lactational Amenorrhea Method." To help you learn more, LLLI's Online Store features the books Breastfeeding and Natural Child Spacing by Sheila Kippley and Your Fertility Signals by Merryl Winstein (also available in Spanish).
More information on this topic may be found at our resource page for Breastfeeding and Fertility.