What causes the menstrual cycle in the luteal phase?

The Menstrual Cycle in the Luteal Phase

The luteal phase of the menstrual cycle is primarily driven by the corpus luteum's production of progesterone and estrogen, which is dependent on luteinizing hormone (LH) stimulation. 1, 2

Hormonal Regulation of the Luteal Phase

Corpus Luteum Formation and Function

• After ovulation, the ruptured follicle transforms into the corpus luteum under the influence of the mid-cycle LH surge 2
• The corpus luteum becomes the primary endocrine structure of the luteal phase, secreting:
  • Progesterone (primary hormone)
  • Estrogen (secondary hormone)
  • Inhibin

Key Hormonal Changes

• During the luteal phase, progesterone and estrogen levels progressively increase until mid-luteal phase, then slowly decline if pregnancy doesn't occur 3
• Progesterone reaches peak levels approximately 8 days after the LH surge 4
• LH from the pituitary serves as the main luteotrophic hormone, maintaining corpus luteum function through the luteal phase 2

Physiological Effects of Luteal Phase Hormones

Progesterone Effects

• Prepares the uterus for potential implantation and embryo development 5
• Maintains the secretory endometrium
• Decreases uterine contractility
• Thickens cervical mucus
• Raises basal body temperature

Estrogen Effects

• Works synergistically with progesterone
• During the luteal phase, high estrogen levels improve glucose metabolism by:
  • Increasing glucose availability
  • Enhancing glycogen storage in skeletal muscle
  • Increasing availability of free fatty acids
  • Promoting oxidative energy metabolism 3

Luteal Phase Duration and Regulation

• The luteal phase begins after ovulation and ends either with:
  • Luteolysis (corpus luteum breakdown) if pregnancy doesn't occur, leading to menstruation
  • Corpus luteum rescue if pregnancy occurs 5
• Normal luteal phase duration is relatively constant at 12-14 days
• Continued tonic LH secretion is essential for corpus luteum steroidogenesis 4

Luteal Phase Deficiency (LPD)

LPD can occur when:

• The secretory activity of the corpus luteum is deficient
• The endometrium responds inadequately to normal progesterone levels 5

Diagnostic findings may include:

• Low mid-luteal progesterone levels (<6 nmol/l) 3
• Decreased luteal phase levels of inhibin and estradiol 6
• Deficient midcycle LH surge 6

Corpus Luteum Fate

If pregnancy doesn't occur:

• Luteolysis (programmed corpus luteum degeneration) occurs through apoptosis
• Progesterone and estrogen levels decline
• Menstruation begins

If pregnancy occurs:

• Human chorionic gonadotropin (hCG) from the implanting embryo "rescues" the corpus luteum
• hCG binds to LH receptors on the corpus luteum, maintaining its function
• The corpus luteum continues to produce progesterone until the placenta takes over this function (luteal-placental shift) 2, 7

Clinical Relevance

• In women with cyclic epileptic attacks, symptoms often occur during the luteal phase when progesterone levels are highest 3
• Polycystic ovary syndrome (PCOS) can disrupt normal luteal phase function through hormonal imbalances 3, 1
• In assisted reproductive technology, luteal phase support is often needed due to disruption of normal LH secretion 5, 8

Understanding the hormonal regulation of the luteal phase is crucial for diagnosing and managing various reproductive disorders and optimizing fertility treatments.