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Progesterone

Progesterone is a steroid hormone crucial for female reproductive physiology. It is produced mainly in the ovaries after ovulation, and in smaller amounts by the adrenal glands and placenta during pregnancy. Progesterone works alongside estrogen to regulate the menstrual cycle, support pregnancy, and influence breast development. Its effects extend to mood, libido, and overall reproductive_health.

Functions

  • Stimulates duct and glandular development in breast_development
  • Regulates the menstrual_cycle by preparing the uterus lining for implantation
  • Modulates uterine sensitivity to oxytocin, helping prevent premature contractions during pregnancy; after birth, decreased progesterone allows oxytocin to trigger labor and postpartum uterine involution.
  • Maintains pregnancy by preventing uterine contractions and supporting the placenta
  • Modulates immune response during pregnancy
  • Influences horniness through modulation of GABA neurotransmission, receptor binding (P4R1), and interactions with other sex steroids; effects may be more pronounced during certain phases like luteal phase.

Detailed Menstrual Cycle Regulation

Progesterone plays a key role in the menstrual cycle. After ovulation (luteal_phase), the corpus luteum secretes progesterone, converting from the follicle after the release of the egg by LH. Progesterone prepares the uterus lining for potential implantation (decidua formation) and inhibits LH surge to prevent immediate re-ovulation if pregnancy occurs. If no pregnancy happens, progesterone levels drop from the corpus luteum after its degeneration during menstruation, along with estrogen. This decline in progesterone is a critical signal for the start of menstruation and the subsequent estrogen rise as follicles develop.

Progesterone-Oxytocin Interactions During Pregnancy

Progesterone exerts an anti-convulsant effect during pregnancy, directly inhibiting uterine contractions via progesterone receptors (e.g., P4R1) in the myometrium. It modulates oxytocin signaling pathways so that oxytocin does not trigger strong contractile responses until delivery. This modulation involves increasing the threshold for oxytocin-induced contraction and altering its release dynamics.

Postpartum Labor Triggering

Following placental delivery, progesterone production ceases significantly because the placenta was a major source during pregnancy. The sharp drop in progesterone levels removes this inhibition of uterine contractility. This decrease allows oxytocin to effectively bind and activate its receptors (OXTR) on uterine muscle cells, promoting strong contractions necessary for the expulsion of the placenta (afterpains) and further involution of the uterus back to its pre-pregnancy state.

Immune Modulation During Pregnancy

Progesterone modulates maternal immune tolerance during pregnancy. It promotes a shift towards T regulatory cells (Th2 cytokines) in the mother, reducing pro-inflammatory responses like those mediated by TNF-α. This anti-inflammatory environment supports fetal implantation and placental development while preventing excessive rejection of the semi-allogeneic fetus. Progesterone also influences leukocyte migration and function at the maternal-fetal interface.

Influence on Mood, Energy Levels

Progesterone affects mood through interactions with GABAergic neurotransmission (acting via allopregnenolone, a neurosteroid) which can have anxiolytic effects. During the luteal phase of the cycle (when progesterone peaks), some women report increased anxiety or depression, potentially linked to estrogen dominance relative to progesterone and its metabolite effects on GABA receptors.

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