What is the primary function of the corpus luteum?

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.

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.