Hormones

Menstrual & Reproductive Cycle Hormones

  • Gonadotropin-Releasing Hormone (GnRH): Secreted by the hypothalamus, it triggers the pituitary gland to release FSH and LH.

  • Follicle-Stimulating Hormone (FSH): Released by the anterior pituitary gland, it stimulates the ovaries to grow and mature an egg (ovarian follicle).

  • Luteinizing Hormone (LH): Also from the anterior pituitary, a sudden "surge" of LH is the trigger that causes the mature follicle to rupture and release the egg (ovulation).

  • Estrogen: Produced primarily by the ovaries (and later the placenta), it builds up and thickens the uterine lining in preparation for a fertilized egg. During pregnancy, it promotes the growth of the uterus and breasts.

  • Prostaglandins: Local chemical mediators that help free the egg during ovulation. They also cause menstrual cramps and help initiate uterine contractions for labor.

Pregnancy & Lactation Hormones

  • Progesterone: Often called the "hormone of pregnancy." Secreted first by the corpus luteum and then by the placenta, it maintains the thick uterine lining and calms the uterine muscle to prevent premature contractions.

  • Human Chorionic Gonadotropin (hCG): Produced by fetal cells and the early placenta, it maintains the pregnancy early on and is the biochemical marker detected by early pregnancy tests.

  • Relaxin: Secreted by the placenta and corpus luteum, it softens the cervix and relaxes the pelvic joints and ligaments to make room for the baby to pass through during birth.

  • Human Placental Lactogen (hPL / hCS): Produced by the placenta, it helps prepare the breasts for lactation and alters maternal metabolism by acting as an insulin antagonist (which leaves more glucose in the blood for the fetus).

  • Prolactin: Secreted by the anterior pituitary, this is the hormone responsible for making breast milk.

  • Oxytocin: Released by the posterior pituitary, it stimulates uterine contractions during labor and after birth. It is also responsible for the milk ejection ("let-down") reflex when a baby breastfeeds.