Why does my hair feel healthier since getting pregnant?

Vitamins. While some women state their hair and nails look better just from being pregnant its generally thought to be from taking prenatal vitamins.
Hair growth cycles. The hair on your body goes through cycles of growth, rest and falling out. The hormones of pregnancy affect these cycles and cause your hairs to grow in sync creating the illusion that your hair is healthier. After delivery, many women notice hair falling out and this is caused by the same synchronization but in a different phase.