Which of the following are characteristics of the menstrual phase of the uterine cycle? Select all that apply.
It is triggered by a sharp decline in estrogen and progesterone
It involves the shedding of the functional layer of the endometrium
Spasms of the spiral arteries occur
It typically lasts from Day 1 to Day 5 of the cycle
It is dominated by rising estrogen levels
Correct Answer : A,B,C,D
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.
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Related Questions
Correct Answer is C
Explanation
Estrogen, specifically estradiol (E2), plays a dual role in the hypothalamic-pituitary-ovarian axis through both negative and positive feedback mechanisms. During the late follicular phase, when estradiol levels remain consistently high—typically >200 pg/mL for at least 50 hours—the feedback on the hypothalamus and anterior pituitary shifts from negative to positive. This stimulates a GnRH surge, which triggers a massive LH release (LH surge), leading to ovulation. The LH surge also initiates the transformation of the ruptured follicle into the corpus luteum.
Rationale for correct answer
C. Estrogen at sustained high levels switches to positive feedback, stimulating GnRH and causing the LH surge. This is the critical hormonal trigger for ovulation around day 14 of a typical 28-day cycle.
Rationale for incorrect answers
A. Progesterone exerts only negative feedback on GnRH, FSH, and LH throughout the cycle, especially during the luteal phase. It does not cause the LH surge and actually prevents further ovulation.
B. FSH is regulated by negative feedback via estrogen and inhibin. It does not trigger the LH surge and is not part of the positive feedback loop leading to ovulation.
D. Inhibin is secreted by granulosa cells and specifically suppresses FSH release via negative feedback. It plays no role in triggering the LH surge or stimulating GnRH.
Take home points
- High sustained estrogen levels trigger positive feedback on GnRH and LH.
- The LH surge causes ovulation and luteinization of the follicle.
- Progesterone and inhibin exert negative feedback and suppress further ovulation.
- FSH supports follicle growth but is not involved in the LH surge trigger.
Correct Answer is C
Explanation
hCG is a glycoprotein hormone secreted by the trophoblast cells of the developing embryo shortly after implantation, typically detectable in maternal serum by day 8–10 post-ovulation. Its primary role is to rescue the corpus luteum, preventing its degeneration and allowing continued secretion of progesterone and estrogen. These hormones maintain the decidualized endometrium, preventing menstruation and supporting early pregnancy. hCG shares structural similarity with LH and binds to LH receptors on the corpus luteum. Peak serum hCG levels reach 100,000–200,000 mIU/mL by 10 weeks gestation.
Rationale for correct answer
C. hCG prevents luteolysis by acting on LH receptors of the corpus luteum, maintaining its progesterone production. This sustains the endometrial lining for implantation and early embryonic development.
Rationale for incorrect answers
A. hCG does not stimulate new follicle development. In fact, high progesterone and estrogen levels maintained by the corpus luteum inhibit FSH, preventing new folliculogenesis during early pregnancy.
B. hCG prevents menstruation by maintaining the corpus luteum and its hormone output. Menstruation is triggered by progesterone withdrawal, which hCG directly prevents.
D. hCG supports the continued production of progesterone, not its inhibition. It ensures the endometrial lining remains intact until the placenta takes over hormone production around week 10–12.
Take home points
- hCG maintains the corpus luteum during early pregnancy.
- It prevents menstruation by supporting progesterone production.
- hCG mimics LH and binds to LH receptors on luteal cells.
- It does not promote follicle development or suppress progesterone.
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