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I’ve seen many textbooks saying that LH surge occurs in the Follicular phase while others say it occurs in the Ovulatory phase.

I’ve tried googling this, only to end up knowing two things: 1) It occurs 2-3 days before ovulation and 2) It occurs in Ovulatory phase.

Doesn’t that two contradict each other?

If you see the hormonal variation graph, LH surge occurs somewhere between follicular phase and Ovulatory phase. This is quite confusing.

So, at what phase does LH surge actually occur?

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Some graphs about the ovarian cycle mention only the follicular phase and luteal phase, and the ovulation just as a point in time, not a phase, for example, here:

enter image description here

Source: Differencebetween.com

In this case, the LH surge starts in the follicular phase and peaks at ovulation or shortly before it.

Some other graphs also show the "periovulatory" phase (or "ovulatory phase" or just "ovulation" as a phase):

enter image description here

Source: Nature - Scientific Reports

In this graph, LH surge starts few days before ovulation, so in the follicular phase and peaks in periovulatory phase.

The first half of the menstrual cycle is comprised by the menstrual and follicular phases during which time estrogen levels are low (menstrual phase) and rise (follicular phase) and ends with the periovulatory phase in which follicular stimulating hormone (FSH) and luteinizing hormones (LH) peak.

Now, these graphs are still confusing, but Merck Manual defines the ovulatory phase by the surge of LH and FSH:

The ovulatory phase begins with a surge in luteinizing hormone and follicle-stimulating hormone levels.

In conclusion, LH surge occurs in the ovulatory phase, if the source mentions it as a separate phase, and if not, it occurs in the follicular phase.

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This pathway makes more sense when viewed in an evolutionary context. Many other animals experience induced ovulation, where various stimuli associated with copulation triggers the release of the egg. For example, in fish and cats, the male may bite the back of the female's neck to help induce ovulation. Whatever stimuli are received, they are pieced together by neurons that respond by secreting GnRH from the hypothalamus. GnRH is routed via the hypophyseal portal system (a blood vessel) directly to the anterior pituitary to trigger LH and FSH production. So what in humans seems like a slow and cyclic hormonal cycle is adapted from a rapid reflex response to sensation.

As a result, the LH and FSH surge does occur very close to the end of the follicular phase. It has the 'intended' result of causing ovulation immediately afterward. The follicle bursts, the egg is released, and by definition the part that remains is the corpus luteum - so the physical act of ovulation starts the luteal phase.

I see there's a rule about citations: see https://en.wikipedia.org/wiki/Induced_ovulation_(animals) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6101186/ http://www.ansci.wisc.edu/jjp1/ansci_repro/lec/lec_25_dog_cat/lec25out.htm

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