"During pregnancy the increased levels of estrogen in your body freezes hair in the growing (or "resting") phase of the cycle. Hair that would normally fall out stays put, resulting in thicker hair. After you give birth and your estrogen levels decline, however, all that hair that was resting starts to fall out. This usually starts the third or fourth postpartum and ends by six months, says Joanne Stone, MD, an ob-gyn at Mount Sinai Medical Center in New York City and coauthor of Pregnancy for Dummies (John Wiley & Sons). But some women say it can last for a year. This temporary hair loss doesn't mean you're deficient in nutrition or vitamins, says Dr. Stone. It's simply hormonal."