Progesterone Relaxes the Myometrium and Prevents Lactation During Pregnancy
The correct answer is C. Progesterone. This hormone maintains uterine quiescence throughout pregnancy by reducing myometrial contractility and suppresses lactation until after delivery when progesterone levels fall.
Myometrial Relaxation Mechanisms
Progesterone keeps the uterus quiescent during pregnancy through multiple well-established mechanisms:
- Reduces contraction frequency in the myometrium by decreasing prostaglandin synthesis and infection-mediated cytokine production 1
- Changes progesterone receptor expression (decreased PR-A/PR-B ratio) which keeps the uterus in a non-contractile state 1
- Counteracts prostaglandin stimulatory activity - adequate progesterone concentrations in myometrium are able to block prostaglandin-induced contractions 2
- Maintains uterine quiescence by limiting production of stimulatory prostaglandins and inhibiting expression of contraction-associated protein genes within the myometrium 3, 4
The evidence clearly demonstrates that progesterone's primary role during pregnancy is maintaining myometrial relaxation. When progesterone is withdrawn at parturition, COX-2 enzyme expression becomes maximal, prostaglandin production is stimulated, and labor begins 1.
Lactation Suppression
Progesterone prevents lactation during pregnancy, and its withdrawal after delivery allows milk production to commence:
- High progesterone levels during pregnancy suppress lactation 5
- After parturition, the withdrawal of placental progesterone (along with estrogen) triggers the cascade of postpartum changes, including initiation of lactation 1
- Progesterone acts as a natural immunosuppressant at high concentrations during pregnancy 5
Why Other Options Are Incorrect
GnRH (Option A) and FSH (Option B) are involved in reproductive hormone regulation but do not directly relax the myometrium or prevent lactation during pregnancy 1.
Estrogen (Option D) actually has opposite effects - it increases COX-2 expression in the myometrium, which promotes prostaglandin production and uterine contractions 1. Estrogen withdrawal (not maintenance) is necessary for lactation suppression to end.
Clinical Relevance
This physiologic principle underlies the therapeutic use of progesterone for preterm birth prevention. The American College of Obstetricians and Gynecologists recommends progesterone supplementation (17-alpha-hydroxyprogesterone caproate 250 mg IM weekly or vaginal progesterone) for women at high risk of preterm birth, specifically because it maintains myometrial quiescence 6, 7, 8.