What happens to the corpus luteum if fertilization does not occur?

Progesterone in the menstrual cycle Progesterone is a steroid hormone produced by the corpus luteum after ovulation during the luteal phase. Its primary role is to prepare the endometrium for implantation. Progesterone maintains the uterine lining, inhibits uterine contractions, and increases basal body temperature. Normal serum progesterone levels vary across the cycle: follicular phase <1 ng/mL, luteal phase 5–20 ng/mL, postmenopausal <0.5 ng/mL. It is also critical in modulating immune tolerance during early pregnancy.

Rationale for correct answers

B. Progesterone converts the endometrium from the proliferative to the secretory phase, enhancing glandular secretion and stromal edema to support implantation.

C. It inhibits myometrial contractions by reducing prostaglandin synthesis and promoting uterine quiescence during the luteal phase, preventing embryo expulsion.

E. Progesterone increases basal body temperature typically by 0.3–0.5°C due to its thermogenic effect on the hypothalamic temperature-regulating center.

Rationale for incorrect answers

A. Endometrial proliferation occurs during the follicular phase and is mediated primarily by estradiol. Progesterone opposes proliferation and promotes secretory transformation.

D. Ovulation is triggered by a surge in luteinizing hormone (LH), not progesterone. Progesterone secretion rises only after ovulation has occurred.

Take home points

• Progesterone maintains a secretory endometrium essential for embryo implantation.
• It inhibits uterine contractility by downregulating prostaglandins.
• Progesterone elevation correlates with the post-ovulatory rise in body temperature.
• Estradiol governs the proliferative phase; LH surge triggers ovulation.

The menstrual cycle is a recurring sequence of hormonal and physiological events typically lasting 21–35 days, with an average of 28 days. It is counted from day 1, which is defined as the first day of menstrual bleeding—the visible onset of endometrial shedding. This marks the start of the follicular phase, during which FSH (normal range: 3–20 IU/L) rises to stimulate follicular development. Bleeding lasts about 3–7 days, and estrogen levels gradually increase to rebuild the endometrium. Ovulation generally occurs around day 14, followed by the luteal phase.

Rationale for correct answer

B. The first day of menstrual bleeding is considered day 1 of the cycle. It marks the beginning of the follicular phase and is used clinically to track ovulation, fertility windows, and menstrual regularity.

Rationale for incorrect answers

A. Ovulation typically occurs around day 14 of a 28-day cycle, not at the beginning. It is a mid-cycle event and not used as the starting point for cycle calculation.

C. The last day of menstruation varies among individuals and does not consistently mark the start of a new hormonal cycle, making it an unreliable reference point.

D. The LH surge precedes ovulation by 24–36 hours and occurs around day 13–14, mid-cycle. It is not the beginning of the menstrual cycle and occurs much later than day 1.

Take home points

• Day 1 of the menstrual cycle is the first day of menstrual bleeding.
• This marks the start of the follicular phase and FSH rise.
• Ovulation and the LH surge occur mid-cycle, not at the beginning.
• Accurate cycle tracking begins with the first day of menstruation.