Which hormones are secreted by the anterior pituitary to regulate the menstrual cycle? Select all that apply.

The endometrial cycle aligns with the ovarian cycle and consists of three main phases: menstrual, proliferative, and secretory. The secretory phase coincides with the luteal phase of the ovarian cycle and spans approximately days 15–28. It is regulated primarily by progesterone (normal mid-luteal: 5–20 ng/mL) secreted by the corpus luteum. Progesterone stimulates the endometrial glands to secrete glycogen and other nutrients, making the endometrium thick, vascular, and receptive to implantation. Without fertilization, progesterone levels fall, leading to endometrial breakdown and the start of menstruation.

Rationale for correct answer

C. The secretory phase is driven by progesterone from the corpus luteum and prepares the endometrium for possible implantation. It occurs during the luteal phase of the ovarian cycle.

Rationale for incorrect answers

A. The menstrual phase coincides with the early follicular phase, not the luteal phase. It involves shedding of the functional layer of the endometrium due to hormone withdrawal.

B. The proliferative phase occurs during the follicular phase of the ovarian cycle. It is estrogen-driven and involves rebuilding the endometrium after menstruation.

D. The regenerative phase is not a standard term in the clinical classification of the endometrial cycle. The correct phases are menstrual, proliferative, and secretory.

Take home points

• The secretory phase matches the luteal phase and prepares the uterus for implantation.
• Progesterone from the corpus luteum drives the secretory changes.
• The proliferative phase aligns with follicular phase and is estrogen-dependent.
• Menstrual phase begins with hormone withdrawal and marks the end of the luteal phase.

Menstrual phase of the uterine cycle The menstrual phase marks the beginning of the uterine cycle and is initiated by a sudden drop in estrogen and progesterone levels due to corpus luteum regression in the absence of implantation. This hormonal decline leads to vasoconstriction of spiral arteries, ischemia, and shedding of the functional layer of the endometrium. Normal menstrual flow lasts 3–5 days, and blood loss averages 30–40 mL. Spiral artery spasms result in tissue necrosis, and rising estrogen levels only commence after menstruation is complete.

Rationale for correct answers

A. The withdrawal of estrogen and progesterone at the end of the luteal phase triggers endometrial breakdown and onset of menstrual bleeding.

B. The functional layer of the endometrium undergoes necrosis and is shed through the cervix and vagina as menstrual fluid.

C. Spiral arteries undergo intense vasoconstriction, leading to ischemia and necrosis of the endometrium, which causes menstruation.

D. The menstrual phase typically spans Days 1 to 5 of the cycle, marking the beginning of the uterine cycle before follicular estrogen rise.

Rationale for incorrect answers

E. Rising estrogen levels occur during the proliferative phase, which begins after menstruation. The menstrual phase is marked by low estrogen and progesterone levels.

Take home points

• Menstrual phase starts with progesterone and estrogen withdrawal.
• Endometrial shedding is due to spiral artery ischemia.
• Menstrual flow usually lasts 3–5 days.
• Rising estrogen levels define the proliferative phase, not the menstrual phase.