Hi K.. I have very thin, fine hair so this postpartum hair loss issue was VERY scary for me! After a lot of reading, I found that each woman may be affected differently but on average hair loss is most apparent between 3-6 months. They say that by 1 year, you should be back to normal. The simple way to explain why this happens is because of the 2 stages hair goes through; growing and dying. Each hair will grow and then die and fall out during your normal hair cycle and then be replaced by a new hair. We don't typically notice the small amount of hairs falling out, but we may notice more loss during high stress, some drugs, cancer, etc. During pregnancy, those lovely baby hormones may make MORE of your hair stay in the growing stage with less falling out. That is where some of us see the full luscious, shiny pregnancy hair. After the baby is born, the hormone levels fall and all that hair that was growing begins to die and fall out. Some women notice more than others.
The most common traits you'll see among the hair of new moms is the thinning at the temples and shortly after, the little stand up hairs at the part and forehead (the new growth coming in). Some women will talk with their stylist about a new cut, some prefer shorter, to disguise the loss or make it less tramatic. Some will opt for hair extensions or different colors or highlight.
Some women notice permanent dramatic changes in texture and amount after having kids. But yes, the hair loss will stop. So take a deep breath, we've all been there, and your little baby. :)