Many hormone levels are affected in the body during pregnancy, with several hormones playing major roles during pregnancy. These include:
Human chorionic gonadotropin hormone (hCG). This hormone is only produced during pregnancy---almost exclusively in the placenta. HCG hormone levels found in maternal blood and urine increase dramatically during the first trimester and may contribute to causing nausea and vomiting often associated with pregnancy.
Human placental lactogen (hPL). This hormone, produced by the placenta, helps in the process of providing nutrition for the fetus and plays a role in stimulating milk glands in the breasts in anticipation of breastfeeding.
Estrogen. This group of hormones is responsible for the development of the female sexual characteristics. Normally formed in the ovaries, estrogen is also produced by the placenta during pregnancy to help maintain a healthy pregnancy.
Progesterone. This hormone is produced by the ovaries and by the placenta during pregnancy. Progesterone stimulates the thickening of the uterine lining in anticipation of implantation of a fertilized egg.