When You're Pregnant, The Extra Hormones Coursing Through Your Body Shift Your Hair Cycle.

Hair normally grows in three phases: active growth, resting, and shedding. During these phases, people typically shed 100 hairs every day. When you're pregnant, the extra hormones coursing through your body shift your hair cycle.
Your hair grows or stays on your head and doesn't shed. This is why your hair seems longer and thicker than usual.

Some research also suggests that hair strands actually thickens during pregnancy. "The diameter of the hair increases. Sometimes, a woman's hair becomes more or less curly during or after pregnancy.
There's a lot of thought about whether hormones during pregnancy can alter the shape of the hair follicle. The shape of the follicle dictates the shape of the hair fiber.

Avoiding Chemicals
If you normally dye, highlight, perm, or relax your hair, you may wonder whether to take a break during pregnancy. Some doctors recommend stopping, while others say that it's OK to continue. Still others say that treatments should be avoided in the first trimester, but they're OK later in pregnancy.
Why the difference of opinion? Very few studies have examined the effects of dyes and other hair chemicals in pregnant women. Still, there's a chance that harsh chemicals may be absorbed through your scalp during treatments and passed along to your growing baby.

"We truly don't know if anything is absorbed internally, but you have many pores that are deep on the scalp, and there's always the potential, especially if the scalp is irritated," says dermatologist Nia Terezakis, MD, a clinical professor at Tulane University. "The fewer chemicals, the better."
If you're concerned about chemicals but need to keep up your appearance for professional reasons, consider getting highlights or using a natural dye.

Heather Woolery-Lloyd, MD, voluntary assistant professor of dermatology at the University of Miami, says, "If you're just covering grays, dyes like henna are good for people who want to go the more natural route."

Skip keratin hair-straightening treatments during pregnancy. They contain harmful formaldehyde.

Pregnancy Hair Maintenance
You shouldn't need to change your shampoo, conditioner, hair spray, gel, mousse, or blow-dry and curling-iron routine while you're expecting, unless you use prescription dandruff shampoo. Talk to your doctor, because some medicated shampoos can't be used during pregnancy. Some pregnant women can become overwhelmed by aromas, and many hair-care products are scented. If the smell of your shampoo or hair spray becomes too strong to tolerate, switch to a fragrance-free version.

Postpartum Hair Drama
Expect your streak of good-hair days to end a few months after your baby is born: All of the hair that stuck around for longer than usual will begin to fall out.

Afterward, your hair will grow normally. Shedding while you're recovering from pregnancy is normal and healthy. It doesn't mean you're going bald.

By webMD