Here's what's going on. Normally, about 85 to 95 percent of the hair on your head is growing and the other 5 to 15 percent is in a resting stage. After the resting period, this hair falls out — often while you're brushing or shampooing it — and is replaced by new growth. An average woman sheds about 100 hairs a day.
During pregnancy, increased levels of estrogen prolong the growing stage. There are fewer hairs in the resting stage and fewer falling out each day, so you have thicker, more luxuriant tresses.
After you give birth, your estrogen levels take a tumble and a lot more hair follicles enter the resting stage. Soon you'll have more hair coming out in the shower or on the brush. This unusual shedding will taper off and your hair will be back to its pre-pregnancy thickness about six to 12 months after you give birth.
By the way, not all women notice dramatic changes in their hair during pregnancy or the postpartum period. Among those who do, it tends to be more obvious among women with longer hair.