Boob Scoop: The return of your period does not mean the end of breastfeeding. During menstruation, breastmilk does not "go bad" or become less nutritious. Some women do notice a temporary drop in milk supply in the days prior to a period and for a few days into one, due to hormonal fluctuations. However, once menstruation begins and hormone levels return to normal, milk supply will boost back up again. Most babies can compensate well for this temporary drop in supply with more frequent nursing.
Sharen Medrano, Yummy Mummy Support Group IBCLC (www.nycbreastfeeding.com)