15.3 - Menstrual Cycle

Role of Hormones in the Menstrual Cycle

• Follicle-stimulating hormone (FSH): Stimulates follicle growth and oestrogen release.
• Luteinizing hormone (LH): Stimulates ovulation, corpus luteum formation and progesterone release.
• Oestrogen: Repairs and thickens the endometrium; stimulates follicle growth; stimulates FSH and LH release before ovulation.
• Progesterone: Makes the endometrium thick, folded and rich in blood vessels; stops FSH and LH release to prevent follicle growth and ovulation.

Hormonal Changes during the Menstrual Cycle

Days 0–5

• Low progesterone and oestrogen levels cause the thickened endometrium to shed; menstruation begins.
• The hypothalamus releases gonadotrophin-releasing hormone (GnRH).
• GnRH stimulates the pituitary gland to release FSH and LH.
• FSH stimulates follicle growth; the primary oocyte develops into a secondary oocyte within the Graafian follicle.
• Growing follicles release oestrogen, which promotes follicle maturation and endometrial repair.
• Low oestrogen levels inhibit FSH and LH release through negative feedback, preventing new follicle growth.

Days 6–14

• Oestrogen peaks on day 12 and stimulates GnRH release through positive feedback.
• High GnRH stimulates increased FSH and LH release.
• LH peaks on day 13, causing ovulation and release of the secondary oocyte on day 14.
• LH stimulates the remaining follicular tissue to form the corpus luteum.

Days 15–21

• LH stimulates the corpus luteum to secrete oestrogen and progesterone.
• Oestrogen and progesterone inhibit FSH and LH release through negative feedback, stopping new follicle growth.
• Progesterone thickens the endometrium and enriches it with blood vessels for implantation.

Days 22–28

• Without fertilisation, decreasing LH causes the corpus luteum to degenerate; oestrogen and progesterone secretion stops.
• The endometrium sheds and menstruation begins.
• Low progesterone and oestrogen remove inhibition of the hypothalamus and pituitary gland; GnRH, FSH and LH are secreted and a new cycle begins.
• With fertilisation, the corpus luteum continues to secrete progesterone and oestrogen, so the endometrium remains thick for foetal growth.

Hormones in Pregnancy and Miscarriage

• The corpus luteum produces oestrogen and progesterone for the first three to four months of pregnancy; the placenta then takes over until birth.
• Progesterone inhibits FSH and LH, so menstruation and ovulation do not occur during pregnancy.
• An oestrogen–progesterone imbalance may cause miscarriage: low progesterone causes the uterus to contract.

Premenstrual Syndrome and Menopause

Premenstrual Syndrome

• Symptoms appear 7–14 days before the first day of menstruation.
• Caused by an imbalance of oestrogen and progesterone.
• Symptoms include fatigue, headache, emotional instability and bad temper.

Menopause

• Occurs at 46–50 years when ovulation and menstruation stop naturally.
• Reduced progesterone and oestrogen secretion reduces ovarian stimulation by FSH and LH.
• The ovaries stop producing ova; conception is no longer possible.
• Symptoms include difficulty sleeping, hot flushes, low mood and bad temper.