What directly causes ovulation during the menstrual cycle?

What Directly Causes Ovulation During the Menstrual Cycle?

The correct answer is B: A sudden increase of LH (luteinizing hormone) directly causes ovulation. This LH surge is the most clearly defined and reliable biochemical marker that triggers the ovulatory process, occurring 16-48 hours before follicle rupture 1.

The Mechanism of Ovulation

The LH surge is the direct trigger for ovulation, with the following temporal relationships 1:

• Ovulation occurs 32 hours (median) after the LH rise begins, with 90% of ovulations happening between 16-48 hours after the initial LH increase 1
• Ovulation occurs 16.5 hours (median) after the LH peak, with most occurring between 8-40 hours after peak levels 1
• The LH surge demonstrates a 2-4 fold increase above baseline levels and lasts 24-30 hours during the preovulatory period 2

Why the Other Options Are Incorrect

Estrogen's role is indirect, not direct 3, 4, 5:

• A sustained midcycle increase in estradiol (option D - gradual increase) causes positive feedback on the hypothalamus, which then increases GnRH secretion 3
• This leads to the LH surge, but estrogen itself does not directly cause ovulation 5
• The estradiol peak actually occurs 24 hours before ovulation, well before the actual follicle rupture 1

Progesterone rises after, not before ovulation (option C is incorrect) 2, 6:

• LH induces a 2-3 fold increase in progesterone production above baseline approximately 12 hours prior to ovulation 2
• However, this rise is relatively small and occurs as a consequence of LH action, not as the cause of ovulation 2
• Progesterone secretion is characteristic of the luteal phase (after ovulation), not the trigger for it 6

Estrogen decrease is a consequence, not a cause (option A is incorrect) 2:

• LH induces a marked reduction in estrogen production approximately 12 hours prior to ovulation 2
• This decrease occurs after the LH surge has already begun 1

Clinical Significance

The LH surge is the most reliable predictor of impending ovulation 2, 1:

• It provides the most clearly defined biochemical marker with a relatively short, predictable preovulatory period 2
• Ovulation detection methods based on LH measurement are the most accurate for timing conception or contraception 1
• The LH surge causes the initiation of the ovulation process by triggering follicle rupture and transformation into the corpus luteum 5