D3.1.19 Hormonal control of pregnancy and childbirth (HL) Notes

Progesterone Keeps Pregnancy Going

1. Progesterone is the pregnancy maintenance hormone.
2. During early pregnancy (weeks 1-9), the corpus luteum secretes progesterone. If secretion stops, the lining sheds and the pregnancy ends.
3. 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 Decline Allows Childbirth to Begin

1. Progesterone doesn't remain elevated indefinitely.
2. As the fetus matures, it signals the placenta to reduce progesterone secretion.
3. Since progesterone inhibits uterine contractions, its reduction removes that inhibition.
4. The uterus can now respond to contractile signals, setting the stage for labor to begin.

Oxytocin Drives Uterine Contractions Through Positive Feedback

1. Once progesterone levels drop, oxytocin from the pituitary gland becomes the primary driver of labor.
2. The diagram shows how this works as a self-amplifying cycle:
   1. Baby's head presses against cervix (bottom of diagram), activating stretch receptors.
   2. Stretch receptors send nerve impulses to the brain (left side).
   3. Brain stimulates pituitary to secrete oxytocin (top), which travels through the bloodstream to the uterus (right side).
   4. Oxytocin triggers uterine contractions that push the baby harder against the cervix, increasing the stretch stimulus.
3. This creates positive feedback, more stretch produces more oxytocin, which produces stronger contractions, which increases stretch further.
4. The cycle intensifies until delivery occurs.
5. Once the baby is born and no longer presses on the cervix, the stimulus disappears and the feedback loop stops.