How do birth control pills work?

Dear Woman, 

These small but mighty pills do many things beyond preventing pregnancy—birth control pills can help regulate the menstrual cycle, minimize the appearance of acne, and reduce period pain. Most birth control pills contain synthetic forms of either progestin (the synthetic version of progesterone, a hormone that naturally occurs in the body) alone or a combination of estrogen and progestin. While the body’s natural hormone levels fluctuate throughout the menstrual cycle, birth control helps stabilize hormone levels to prevent pregnancy. To learn more about the different types of birth control pills, how they prevent pregnancy, and considerations about who should and shouldn’t take them, read on. 

It might be helpful to first introduce the key players in the body’s natural menstrual cycle.  Follicle-stimulating hormone (FSH) stimulates follicles that contain eggs. Luteinizing hormone (LH) stimulates the release of a mature egg from the ovaries to be fertilized by sperm. High levels of progesterone and estrogen also create a proper home for a fetus if a pregnancy happens. If you’re interested in learning more about these hormones, feel free to check out a guide to the menstrual cycle.  

When you’re on birth control the traditional menstrual cycle doesn’t run as normal. Instead, the synthetic hormones overpower the natural ones. Synthetic estrogen suppresses FSH to prevent follicle formation and progestin suppresses LH to prevent the release of egg during ovulation. When ovaries don't release eggs, sperm can't fertilize them to result in pregnancy. Even if an egg is released, progestin thickens cervical mucus so that there’s a barrier against sperm entry.  

When it comes to birth control pills, there are two main types—progesterone-only pills (POP), which only contain progestin, and combined oral contraceptive (COC) pills, which contain estrogen and progestin.  

There are two main forms of POP, the minipill and drospirenone (brand name Sylnd) . The minipill has 28 active pills, which means you take one every day for 28 days. Because of this, many minipill users stop having monthly withdrawal bleeding. Additionally, each pill must be taken in the same three-hour window every day. If you miss a dose, you should choose alternative contraception or avoid sex for two days. The other POP is drospirenone. Drospirenone, is active for longer than the minipill’s progestin giving you 24 hours before you’re not protected against pregnancy. There are 24 active pills with four days of inactive placebo pills in packs of these pills. You might encounter withdrawal bleeding during days when you’re not taking hormones (this would be during the four days on inactive placebo pills). 

Now onto COC pills, of which there are four different types that vary by length and how the hormones in the pills are dispersed.  

• Conventional COC pills generally have either 21 active pills and seven inactive pills or 24 active pills and 4 inactive pills, creating packs of 28 total pills; this results in withdrawal bleeding once per month.  
• Extended cycle COC pills usually contain 84 active pills, sometimes followed by seven inactive pills; this results in withdrawal bleeding about once every three months. 

In addition to differences in how many active and inactive pills there are in a pack, there’s variation in hormones levels among the active pills:  

• Monophasic active pills contain the same amount of hormones in every active pill. 
• Multiphasic active pills have varying levels of hormones throughout the 28-day cycle, which is supposed to reduce the side effects related to hormones. 

With this information, you might be wondering if birth control’s right for you. People may choose COCs if they are experiencing severe menstrual cramps, heavy bleeding, PMS symptoms, or unwanted hair growth from polycystic ovary syndrome (PCOS). Some people might opt for POPs if they’re already predisposed to conditions that the COC can lead to, including blood clots, migraines, and high blood pressure. Yet for other individuals, hormonal birth control may not be a good option. For example, your doctor may discourage use of the minipill if you a history of or currently have breast cancer, certain liver diseases, unexplained uterine bleeding, or take anticonvulsant or anti-tuberculous agents. Your doctor may discourage use of COC pills if you have certain pre-existing conditions or a family history of certain diseases. It’s best to speak with a health care professional about making your decision. After sharing your medical history, lifestyle, and preferences, they should be able to recommend what kind of birth control is safest for you. Hopefully speaking with them will clear up any concerns. 

Take care,