Hormonal Contraception: Birth Control Pills, Rings, Implants, etc.
Before going to medical school, I was always confused about how hormonal birth control worked. I felt like doctors always just said "it tricks your body's hormones and prevents pregnancy." If you are curious by nature like myself, this answer does not suffice!
There are two main categories of hormonal contraception:
1. Combined (contains estrogen and progesterone)
Both categories take various different forms, such as pills, patches, rings, implants, etc.
1. Combined hormonal contraceptives work by stopping the growth of a follicle. It does this by basically screwing with the body's natural pulsatile rhythm of LH and FSH hormone secretion from the brain. By adding more estrogen and progesterone into the bloodstream, combined hormonal contraception puts natural hormone feedback loops to a stop. The result of this is consistently low LH and FSH, and therefore a follicle cannot develop or ovulate.
Another added effect of combined hormonal contraceptives is that the progesterone in the medication causes the normal mucous in the cervix to become thicker, which makes it more difficult for sperm to get through the cervix up into the uterus.
At this point you might be wondering what I always wondered: why do women (who don't have POF) go through menopause at a normal age even if they were on the pill for 20 years? Wouldn't this delay menopause because no eggs are getting released? The answer is no, because eggs are still disintegrating in the ovary at the same rate whether a woman is on hormonal contraception or not. Weird, isn't it?
At this point you also might be wondering how women on combined hormonal contraceptives have a period during their placebo week (if they are taking pills) or when they remove their vaginal ring, etc. This period is induced by the drop in progesterone after removing the hormones.
2. Progesterone-only hormonal contraceptives work differently depending on how high the dose is. Low dose progesterone-only contraceptives prevent follicle development and ovulation in only approximately 50% of the cycles, but they thicken cervical mucous and prevent sperm from entering the uterus in this way. Though it may not sound like enough protection, even these lower-dose progesterone contraceptives are over 99% effective at preventing fertilization. Medium and high dose progesterone-only contraceptives mess up the natural pulsatile rhythm of LH and FSH and therefore prevent follicular development and ovulation, and they also have the same mucous effects.