There’s tons of charts showing how hormone levels, uterine lining, and body temp shift over the course of a normal cycle but nothing showing what being on the pill does to those factors.
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The feedbacks involved are complicated. But the simple version for the most common types of hormonal birth control is that they prevent the spike of luteinizing hormone (“LH”) that causes ovulation (the release of a fertile egg cell from the ovaries).
More specifically, progestins (a class of hormones that includes progesterone) suppress the production of follicle-stimulating hormone (“FSH”), and suppressing FSH in turn suppresses the production of LH. During a normal menstrual cycle, progestins spike just after ovulation and push LH and FSH levels back down, which prevents the release of additional eggs in the event that pregnancy occurs. But when on birth control, which typically contains progesterone or other progestins, FSH never spikes, so LH never spikes, so ovulation either doesn’t happen or is less likely to happen.
There are other factors, too. For example, most birth controls thicken cervical mucus, which is a coating on the inside of the cervix that is basically there to allow sperm to “swim” up to the eggs to fertilize them. Thickened mucus slows the sperm down, reducing the chance they reach the egg if one does happen to be released.
The uterine lining builds up starting when you start the month’s pill. Not as much, usually, as it would normally build up, but a bit. It gets thicker throughout the month and then sheds, in a process called a progesterone withdrawal bleed, when you stop the pills and start the placebo week.
As for hormone levels: there are 3 kinds of pills. One kind gives you the same hormones every day and then you have a placebo week. So your estrogen and progesterone will stay pretty much the same every day you use those pills. Those levels will be low normal. Like the peaks and valleys of normal cycling are at an end, and you stay at a low maintenance level all the time. Your LH and FSH will be suppressed. Your testosterone will be suppressed all the time as well. During the placebo week, estrogen and progesterone will bottom out initially, and then estrogen might rise a little bit near the end of the week, but will go down after you resume the pills. Your progesterone will stay low that week. If you’re using the second kind of pill, which is same hormones every day and no placebo week, then the above is true but you never have the drop of the placebo week. Your uterine lining gets thick but the continuous pills with no placebo typically have low amounts of estrogen and progesterone, so your lining will shed a little mostly likely but eventually a state will happen where your lining no longer sheds, it stays relatively thin all the time. The other possibility is a multiphase pill that features different amounts of estrogen and progesterone in a few phases throughout the month. Your estrogen and progesterone will rise and fall based on the dose of a given day, but the fluctuations aren’t huge and it’s pretty similar to the 3 weeks of same hormones and one week of placebo.
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