In pregnancy, your body's hormones are responsible for growing and sustaining your baby's life (yay!), as well as the less fun symptoms and side effects like nausea, dizziness, reflux, and skin issues (boo!). These hormones, specifically the high levels of estrogen (the hormone that's also responsible for transferring nutrients and supporting your growing baby), are also responsible for changes in your hair -- for better and worse.
Most people experience hair changes in pregnancy, some more subtle than others. There are three phases of hair growth: the active growth phase (2-6 years long, called "anagen"), transitional phase (2-3 weeks long, called "catagen"), and the resting phase before falling out at around 100 days (called "telogen"). In pregnancy, the extra estrogen causes an extended growth phase for your hair. Because this growth phase is prolonged, hair that would normally pass through this phase and fall out, is retained. Outside of pregnancy, you shed 25-100 hairs every day.
What this means for a lot of people is that pregnancy hair feels thicker and more full -- because it is! Some people even report their hair being more shiny, and changing in texture (from curly to more straight or vice versa). What this also means is that postpartum, when your estrogen levels drop off and steadily return to normal, you will start to shed hair. A lot of hair. The peak of post-birth hair shedding is around 3-6 months postpartum, and can last up to 12 months. During phase, it can seem as though you're losing all of your hair! It's not uncommon for clumps or a brush full of hair to come out at once. Don't worry though; you aren't actually losing all of your hair. Really, it's hair that should have been lost progressively during the 10 months of pregnancy, and now it's coming out in rapid succession. When your postpartum hair shedding is complete, your hair may appear thin or sparse/bare in places. Once you begin the normal growth pattern, those areas will fill back in and return to normal. In fact, many people experience the postpartum hair regrowth phenomenon where lots of hair grows back in around the same time, so there are many tiny new hairs sticking up, making your hair appear frizzy at times.
For most people, by the end of your first year after birth, normal hair growth returns. Though it can take a little while for everything to return to how your hair looked prepregnancy. Losing your hair can feel distressing. It helps to know that this is a natural process, almost everyone experiences it, and it is only temporary.