What is the primary role of estrogen in the menstrual cycle?

Estrogen function in the menstrual cycle
Estrogen is a key steroid hormone secreted primarily by the granulosa cells of the developing follicle. It plays a central role in the follicular phase of the menstrual cycle by stimulating endometrial proliferation, enhancing FSH receptor expression in follicles, and exerting negative feedback on FSH and positive feedback on LH as ovulation approaches. Normal estradiol (E2) levels range from 30–400 pg/mL, peaking around 200–300 pg/mL just before ovulation. Estrogen also supports cervical mucus thinning to facilitate sperm transport and primes the endometrium for possible implantation.

Rationale for correct answer

C. Estrogen stimulates mitotic activity and proliferation of the endometrial functional layer during the follicular phase. This prepares the uterine lining for potential implantation post-ovulation.

Rationale for incorrect answers

A. Ovulation is triggered by a surge in LH, not directly by estrogen. Estrogen contributes indirectly by reaching a threshold that induces the LH surge, but it does not trigger ovulation by itself.

B. Maintenance of the corpus luteum is supported by LH and, if pregnancy occurs, human chorionic gonadotropin (hCG). Estrogen does not sustain corpus luteum function.

D. Estrogen initially inhibits FSH secretion via negative feedback, but this is not its primary role. Its main function is endometrial proliferation and preparing the body for ovulation.

Take home points

• Estrogen stimulates the endometrial lining to proliferate during the follicular phase.
• LH, not estrogen, is the primary trigger for ovulation.
• Corpus luteum maintenance depends on LH or hCG, not estrogen.
• Estrogen has a dual feedback effect on FSH depending on its concentration and timing.

The menstrual cycle is divided into the menstrual, follicular, ovulatory, and luteal phases, regulated by hormonal fluctuations of estrogen, progesterone, FSH, and LH. The menstrual phase begins on day 1 of the cycle and lasts about 3–7 days, during which the functional layer of the endometrium is shed due to a sudden drop in progesterone (from normal mid-luteal values of 5–20 ng/mL to <1 ng/mL) after corpus luteum regression. FSH (normal: 3–20 IU/L) begins to rise to recruit new follicles. Blood loss ranges from 30–80 mL, and prostaglandins contribute to uterine contractions and cramping.

Rationale for correct answer

D. The menstrual phase involves the shedding of the functional layer of the endometrium due to the withdrawal of progesterone and estrogen. It marks the beginning of a new cycle.

Rationale for incorrect answers

A. The follicular phase follows menstruation and involves regeneration of the endometrium under the influence of rising estrogen levels from developing follicles.

B. Ovulation is the release of an ovum due to the LH surge, around day 14. The endometrium is not shed during this phase; it continues to thicken in preparation for implantation.

C. The luteal phase occurs after ovulation and is marked by progesterone secretion from the corpus luteum to maintain the endometrium. Shedding occurs only if implantation does not happen.

Take home points

• The menstrual phase is when endometrial shedding occurs due to hormonal withdrawal.
• Estrogen and progesterone drop sharply, triggering menstruation.
• The follicular phase is for endometrial regeneration, not shedding.
• Luteal phase maintains the endometrium; shedding starts only if it ends.