A good reference for anyone concerned about this normal phenomenon post
birth is Blackburn and Loper (1992). Maternal, Fetal, and Neonatal
Physiology: A Clinical Perspective. W. B. Saunders.

She states:

" Under the influence of estrogen dring pregnancy, the rate of hair
growth slows and the anagen (growth) phase is prolonged. This results in
an increased number of anagen hairs and a decrease in telogen (resting
stage) to less than 10% during the 2nd and 3rd trimesters. During the
postpartum period, these anagen hairs enter catagen then telogen and are
shed. Since there are more anagen hairs than usual, most postpartum women
experience a increased hair loss beginning 4 to 20 weeks following
delivery. During this time 30 to 35% of the hairs may enter telogen.
Generally, complete gregrowth occurs by 6 to 15 months, although the hair
may  be less abundant than prior to pregnancy." (p497-498).

Ione Sims, CMN, IBCLC