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.
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