Questions: Reproductive System Anatomy and the Hormonal Cycle

Question 1

During the follicular phase, rising estrogen levels initially suppress FSH and LH secretion. Yet just before ovulation, surging estrogen triggers a massive LH spike. What explains this reversal?

• A. Estrogen switches from negative to positive feedback on the pituitary once it exceeds a sustained threshold (~200 pg/mL for ~36 hours), triggering the LH surge
• B. A separate hypothalamic surge center activates on day 14 of every cycle, independent of estrogen levels
• C. The follicle physically stimulates the ovary, which directly causes LH release through a local reflex
• D. Progesterone from the corpus luteum triggers the LH surge by overriding the estrogen-based negative feedback

Answer: (A)

The LH surge is one of the most important positive feedback loops in physiology. For most of the follicular phase, estrogen exerts negative feedback on the pituitary, suppressing gonadotropin release. When the dominant follicle matures sufficiently, estrogen rises above ~200 pg/mL and is sustained there for approximately 36 hours — this crosses a threshold that switches the pituitary response from suppression to amplification. The pituitary now releases a massive surge of LH that ruptures the dominant follicle and triggers ovulation. The corpus luteum does not yet exist before ovulation, ruling out option D.

Question 2

A sexually active woman receives a positive home pregnancy test 10 days after ovulation. What is the test detecting, and what does it tell us about what has happened biologically?

• A. Progesterone — elevated progesterone indicates the luteal phase is extended, which correlates with pregnancy
• B. LH — the LH surge persists through early pregnancy and is detected by home tests
• C. hCG — the embryo has implanted and is actively secreting human chorionic gonadotropin to rescue the corpus luteum
• D. Estrogen — the placenta begins estrogen production at implantation, which home tests detect

Answer: (C)

Home pregnancy tests detect hCG (human chorionic gonadotropin), a hormone secreted specifically by the embryo/trophoblast after successful implantation. hCG acts like LH to rescue the corpus luteum from the regression that would normally occur ~14 days after ovulation, maintaining progesterone production and preventing menstruation. A positive test therefore confirms not just fertilization but successful implantation — the embryo is actively signaling its presence to the maternal endocrine system.

Question 3

The LH surge that triggers ovulation operates through the same negative feedback mechanism that governs GnRH, FSH, and LH throughout the rest of the menstrual cycle.

• T. True
• F. False

Answer: False

The LH surge is a rare example of positive feedback in physiology. Throughout most of the cycle, the HPG axis runs on negative feedback: elevated estrogen and progesterone suppress GnRH and gonadotropin secretion — the standard regulatory pattern. Near ovulation, estrogen crosses a sustained threshold and the pituitary response switches: instead of suppressing LH release, high estrogen now stimulates it, producing the LH surge. Positive feedback loops are unusual in physiology because they amplify rather than stabilize — they are appropriate here precisely because ovulation requires a brief, explosive signal rather than steady-state regulation.

Question 4

A positive pregnancy test detects hCG because the embryo secretes this hormone to prevent the corpus luteum from regressing, thereby maintaining progesterone production.

• T. True
• F. False

Answer: True

After ovulation, the corpus luteum normally regresses after ~14 days if no pregnancy occurs, causing progesterone and estrogen to fall and triggering menstruation. If the embryo implants, it secretes hCG, which acts on LH receptors in the corpus luteum to maintain its function — progesterone production continues, the endometrium is preserved, and menstruation is prevented. This is the embryo's first hormonal signal to the mother, making hCG the earliest detectable marker of pregnancy. The placenta eventually takes over progesterone production around week 10.

Question 5

Explain why the LH surge is described as a positive feedback mechanism and why positive feedback is unusual in reproductive physiology.

Model answer: Positive feedback occurs when a signal amplifies its own production rather than suppressing it. During the follicular phase, estrogen initially suppresses LH release (negative feedback, which maintains stability). When estrogen rises above ~200 pg/mL and is sustained for ~36 hours, the pituitary switches response and begins secreting more LH in response to the high estrogen — the output amplifies the input until the follicle ruptures. Positive feedback is unusual in physiology because it is inherently destabilizing: left unchecked, it would produce runaway amplification. It is appropriate here because ovulation requires a brief, explosive LH surge rather than a steady signal — after the follicle ruptures, estrogen drops, and the feedback loop self-terminates.

Understanding this switch from negative to positive feedback at a threshold is essential for understanding the timing of ovulation. It also explains why cycle length is variable: the timing of the LH surge depends on when the dominant follicle produces sufficient sustained estrogen, which varies across cycles and individuals — making the '14 days' rule an approximation.