I've been trying to figure out through online sources (I don't own any medical textbooks) what the exact mechanism behind the birth control pill (henceforth "the pill") is, and I am often finding contradicting information. This surprises me, since so many women take hormonal birth control that what it does should be known and accessible to everyone.
The following is gathered from PBS and Harvard. As I understand it, the female menstrual cycle is controlled by several organs working in feedback loops: after menstruation, there is a low concentration of estrogen and progesterone in the bloodstream, impelling the hypothalamus-hypofysis to secrete FSH and LH. This stimulates the maturation of a single follicle in the ovaries. This follicle secretes estrogen, signalling to the hypothalamus-hypofysis duo to stop producing FSH and LH. With enough estrogen, a sudden burst of FSH and LH is secreted, causing ovulation. The rest of the cycle is high in progesterone, and by lack of implantation, progesterone production slows and we are back at menstruation.
The goal of the pill is to prevent ovulation (rather than prevent implantation) by playing into these feedback loops.
A first confusion is as follows: I know the pill contains estrogen, yet (as both sources attest to) it is supposedly the high levels of estrogen that induce the FSH/LH spike that triggers ovulation. So then, why wouldn't taking exogenous estrogen cause ovulation rather than preventing it? You are essentially replacing the follicle by a blister pack.
A second confusion is that FSH and LH are so often mentioned in the same breath (i.e., you'll see "FSH" and "LH" less than "FSH and LH") that it is unclear to me what their separate role is. On this ScienceDirect page, in the only exercept that mentions "CHC", it is mentioned that we should consider FSH and estrogen as one loop, and LH and progesterone. The former causes maturation of follicles, the latter causes ovulation (maturation of lutheal cells). This is the only place I have found this distinction, and I'm unsure if it is true; in any case, the above point of contention still applies, because as the pill also contains progesterone, taking exogenous progesterone would cause ovulation.
Besides, I am aware that there exist progesterone-only pills. I'm not sure if they use the same mechanism (this thread is too unclear), because if they do, I don't see the point of having the combined pill contain estrogen.
According to Healthy Women, it is actually the change in estrogen rather than a stable level which causes ovulation. (Rephrasing this as an engineer, the hypothalamus-hypofysis duo detects a high derivative rather than a high baseline in order to trigger the FSH/LH spike.) This seems to be confirmed by Cleveland Clinic in saying
"Chronic (long-term) estrogen exposure inhibits FSH release, whereas rising estrogen levels, which normally occur before ovulation, trigger your pituitary gland to increase levels of gonadotropins (FSH and LH)."
On the other hand, the Crash Course Biology video at 6:02 says in a pop-up that FSH and LH are inhibited by low estrogen but boosted by high estrogen. So, we have a third confusion: does the pill mechanism play on the hormone level, or its rate of change?
So far, I've hypothesised two possible pill mechanisms:
- Pill keeps estrogen constant, which means the derivative is 0, so the FSH/LH spike isn't triggered (and their initial boost after menstruation is triggered due to GnRH from the hypothalamus which is secreted due to the drop in estrogen due to placebo pills, mimicking menstruation).
- Placebo pills during menstruation cause very low estrogen. This causes GnRH, which causes FSH. Then the pill saturates the body with estrogen to make it seem like a follicle is growing (above a lower threshold), causing FSH to stop, yet a real follicle would keep increasing estrogen (above a higher threshold) to trigger the spike during ovulation.
I don't like either explanation. There should be a clear mechanism that disambiguates all of the above confusions and comports with all sourced cited above (assuming they're all correct), and comports with all the styles of pill (combined vs. progesterone-only).