How does the birth control pill work?

If you’re considering birth control for the first time, or you’re looking to switch up the type of birth control you already have, finding the type of contraception that’s right for your body can feel like a daunting process.
From the implant to the IUD, there’s a wide range of contraceptive options out there. Ultimately, having a conversation with your doctor about birth control options can help you decide what’s best for your sexual and reproductive health.
In conversation with experts, we’ll break down what you need to know about the most commonly prescribed type of contraception in the United States: the pill.
“The most common and most familiar form of prescription birth control are birth control pills,” says Dr. Jennifer Robinson, MD, MPHTM, PhD, an obstetrician/gynecologist and assistant professor in Gynecology and Obstetrics at the Johns Hopkins School of Medicine. 
“The birth control pill is a daily hormone-based medication that’s used by a person with ovaries and a uterus to prevent pregnancy,” says Dr. Gina Frugoni, MD, an assistant professor of Obstetrics, Gynecology, and Reproductive Sciences at UC San Diego School of Medicine and obstetrician/gynecologist at UC San Diego Health.
The birth control pill comes in two forms: the combined oral contraceptive pill and the progestin-only pill (also known as the minipill). The biggest difference between the two are the hormones they contain. The combination pill is made up of estrogen and progestin, whereas the progestin-only pill only contains progestin, per Healthline. 
The combination pill is the most commonly prescribed type of oral contraceptive, Robinson says. Though less common, the progestin-only pill can be prescribed if you’re breastfeeding, concerned about taking birth control with estrogen, or if you’re at risk for blood clots, high blood pressure or heart problems, per Mayo Clinic. 
“Each birth control method, for the most part, has multiple mechanisms for how to prevent pregnancy,” says Robinson.
The combination pill prevents pregnancy by stopping ovulation. When you take the pill, “hormones temporarily give a signal to the brain that no ovarian stimulation is needed,” preventing the body from releasing an egg, Frugoni says. If there’s no egg, no pregnancy can occur. 
Secondly, the combination pill will prompt the body to thicken the cervical mucus, creating a barrier that “interferes with how well sperm function,” Robinson says.
The progestin-only pill also prevents pregnancy by thickening the cervical mucus, per Mayo Clinic. However, key differences exist between the two pills.
While progestin can stop ovulation from occurring, it isn’t consistent. Four in 10 women continue to ovulate while taking the progestin-only pill, according to The American College of Obstetricians and Gynecologists. The progestin-only pill also works to thin the endometrium, making it more difficult for an egg to implant into the uterus, per Healthline.
Possible side effects of taking the combination pill include sore breasts, nausea, headaches and spotting, according to ACOG. Rare, serious side effects of the combination pill are blood clots, strokes or heart attacks. It is not common, but still possible to develop high blood pressure from taking the pill, per the FDA. 
More:What is an IUD? Answering the birth control questions you were too afraid to ask
According to the FDA, possible side effects linked to the progestin-only pill include acne, sore breasts, nausea, headaches, irregular vaginal bleeding and weight gain.