Which hormone, when at consistently high levels for a sustained period, switches from negative to positive feedback on GnRH and LH, leading to the LH surge?

The secretory phase follows ovulation and spans days 15–28 of a typical 28-day menstrual cycle. It is driven by progesterone secreted by the corpus luteum. Progesterone transforms the previously estrogen-primed proliferative endometrium into a thick, vascular, and glandular lining capable of supporting implantation. Endometrial glands become coiled and secrete glycogen-rich fluid to nourish a potential embryo. Progesterone levels typically peak at 5–20 ng/mL in the mid-luteal phase. If implantation does not occur, progesterone falls sharply, leading to menstruation.

Rationale for correct answer

D. Progesterone is the main hormone that prepares the endometrium during the secretory phase. It induces glandular secretion, stromal edema, and vascularization to support early pregnancy if fertilization occurs.

Rationale for incorrect answers

A. Estrogen plays a primary role in the proliferative phase, promoting regeneration of the endometrium but not in its secretory transformation. It works before ovulation, not after.

B. FSH stimulates follicular development and estrogen secretion during the follicular phase, but it does not act on the endometrium directly and is not dominant in the luteal/secretory phase.

C. LH triggers ovulation and supports the formation of the corpus luteum, which secretes progesterone, but LH itself does not directly act on the endometrium during the secretory phase.

Take home points

• Progesterone transforms the endometrium into a receptive state during the secretory phase.
• Estrogen prepares the endometrium in the proliferative phase, not the secretory.
• FSH and LH regulate ovarian activity but have no direct endometrial action in the luteal phase.
• Secretory changes include gland coiling, glycogen secretion, and stromal edema.

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.