Progesterone Keeps Pregnancy Going
- Progesterone is the pregnancy maintenance hormone.
- During early pregnancy (weeks 1-9), the corpus luteum secretes progesterone. If secretion stops, the lining sheds and the pregnancy ends.
- After week 9, the placenta takes over progesterone production and continues this for the remainder of pregnancy. If placental progesterone levels fall prematurely, miscarriage occurs.
Progesterone is the "don't contract" signal to the uterus.
Progesterone Decline Allows Childbirth to Begin
- Progesterone doesn't remain elevated indefinitely.
- As the fetus matures, it signals the placenta to reduce progesterone secretion.
- Since progesterone inhibits uterine contractions, its reduction removes that inhibition.
- The uterus can now respond to contractile signals, setting the stage for labor to begin.
Oxytocin Drives Uterine Contractions Through Positive Feedback
- Once progesterone levels drop, oxytocin from the pituitary gland becomes the primary driver of labor.
