Most birth control pills, also called “combined oral contraceptives”, do contain estrogen. These birth control pills contain both a synthetic estrogen and a synthetic progesterone, in dosages that vary by brand.
However, there is one type of birth control pill, the progestin-only birth control pill, or “mini pill”, that does not contain estrogen. This is useful for women who can not tolerate the side effects of estrogen pills, but there are some downsides to taking the progestin-only pill.
For instance, the mini pill needs to be taken at the exact same time every day to be effective, and the failure/accidental pregnancy rate is higher than that of the normal birth control pill. The regular birth control pill can still prevent pregnancy if the pill is missed for less than twelve hours, but the mini pill is ineffective if it is missed for over three hours.
When birth control pills contain both estrogen and progesterone, they more closely mimic a woman’s natural menstrual cycle while effectively preventing ovulation and pregnancy. For most women, the side effects of taking birth controls are very minor, temporary, or nonexistent, and the combined oral contraceptive is a better choice than the mini pill for effective birth control. Women who want to take birth control pills while breastfeeding, however, may want to use the mini pill.
The reason that combined oral contraceptives contain estrogen as well as progesterone is that estrogen improves control of the menstrual cycle and reduces the occurrence of spotting or breakthrough bleeding. Estrogen also contributes to inhibiting ovulation through negative hormonal feedback.
Combined oral contraceptives containing both hormones can also be used to treat other hormonally-dependent conditions, such as polycystic ovary syndrome, dysmenorrhea, or painful menstruation, and endometriosis. Progestin-only mini pills do not have this added benefit, and it is unclear whether mini pills have the same effect as combined oral contraceptives in reducing the occurrence of ovarian and endometrial cancer.
Use of birth control pills containing both estrogen and progesterone is more common than use of the progestin-only mini pill. In most cases, women do not experience significant side effects while taking the estrogen-containing birth control pills, so the regular birth control pill is a more effective contraceptive with slightly more room for error.