By Alice || Edited by Go Ask Alice Editorial Team || Last edited Jul 30, 2021
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Alice! Health Promotion. "How do birth control pills work?." Go Ask Alice!, Columbia University, 30 Jul. 2021, Accessed 26, May. 2024.

Alice! Health Promotion. (2021, July 30). How do birth control pills work?. Go Ask Alice!,

Dear Alice,

How do birth control pills work?

— Woman

Dear Woman, 

Despite being small in size, birth control pills pack a punch in preventing pregnancy! This occurs through several mechanisms, mainly by stopping ovulation; when ovaries don't release eggs, sperm can't find and fertilize them to result in a pregnancy. Most birth control pills contain synthetic forms of one or more hormones: estrogen and progestin. While the body’s natural hormone levels fluctuate throughout the menstrual cycle, taking birth control helps stabilize the body at consistent levels and prevents the usual mid-cycle peaks. Without that mid-cycle estrogen bump, the pituitary gland doesn't release the follicle stimulating hormone (FSH); without FSH, the body doesn’t ovulate. Meanwhile, synthetic progestin works to stabilize mid-cycle peaks in FSH and luteinizing hormone (LH) in order to prevent an egg from being released, and progestin also helps make the uterine lining inhospitable to a fertilized egg, partially limits the sperm's ability to fertilize the egg, and thickens the cervical mucus to hinder sperm movement. Want to know more? Keep on reading! 

There are two main kinds of hormonal birth control pills, which contain different amounts of progestin and estrogen. The first is called a combination pill, which contains both estrogen and progestin. Combination pills can further be broken down into two subcategories, which differ in how frequently users have withdrawal bleeding (which mimics a menstrual period). Conventional pills generally have 21 or 24 active pills and 7 or 4 inactive pills, respectively, creating packs of 28 total pills; this results in withdrawal bleeding once per month. For continuous dosing or extended cycles pills, the packs usually contain 84 active pills, sometimes (though not always) followed by 7 inactive pills; this results in withdrawal bleeding about once every 3 months. Pregnancy protection still exists during the inactive pill window, provided that the next set of active pills are started on time and taken consistently and correctly. Missed doses are more likely to cause birth control failure when they occur during the first week of a new set of active pills. With either form of combination pill, the inactive pills give the body a break and trigger withdrawal bleeding; if using a formulation with only active pills, the withdrawal bleeding often stops completely, though users may still experience some infrequent spotting. On top of being separated into conventional and continuous dosing, combination pills are also categorized by hormone dosage. The monophasic active pills contain the same amount of estrogen and progestin in every active pill, while multiphasic active pills have varying levels of estrogen and progestin throughout the 28-day cycle.  

The second kind of hormonal birth control pill is the progestin-only pill (also known as the minipill). The amount of progestin in the minipill is less than in the combined pill. It works by thickening the cervical mucus, thinning the endometrium, and sometimes suppressing ovulation. The progestin-only pills come in 28-day pill packs, and every pill is an active one. Because of this, many minipill users stop having monthly periods or withdrawal bleeding, though breakthrough bleeding and spotting is more common with minipills than with combination pills. 

Both combination and progestin-only pills are available in several brands, and each has a slightly different blend of hormones. All birth control pills work most effectively when taken daily, but the consequences of not taking the pill at the same time every day differ based on pill type. Combination pills need to be taken every day but not necessarily at the same exact time every day, so it’s no cause for concern if you’re a few hours late in taking a combination pill. On the other hand, minipills must be taken within the same three-hour window of time every day for maximum effectiveness. Regardless of the kind, when you forget your pill (or take a minipill more than three hours late), this causes a dip in your body's levels of the birth control hormones. This may cause the body to ovulate and potentially result in pregnancy. To maximize pregnancy prevention potential, you might consider your day-to-day activities to determine how you could build taking your birth control into already existing routines — maybe after you brush your teeth in the morning, with a set mealtime, or at the same time as another medication. If you’re using birth control pills and are concerned that you’ve missed a dose, it’s recommended that you check the instructions on your birth control packet or consult with a health care provider. The appropriate course of action depends on the type of birth control, the number of pills missed, and where you are in the cycle. 

As if there aren’t already enough options to consider, hormonal birth control is also available in more than just pill form! The combination formula is available as a patch and a vaginal ring, while the progestin-only formula is available as intramuscular shots, an implant, and intrauterine devices (IUDs). Some people may prefer these other forms of hormonal birth control because they can be taken less often (and consequently are easier to not miss a dose). However, for some people, hormonal birth control may not be an option due to various medical conditions or undesirable side effects. For other people, some brands of hormonal birth control might give them side effects while other brands don't. Some may also opt to use birth control due to its non-contraceptive benefits, such as regulating the menstrual cycle, treating acne, and minimizing period pain, among others. Figuring out what works best may take some trial and error, and it’s actually pretty common to try out several types of birth control before settling on one. 

Your health care provider may also offer more information about different kinds of birth control, including helping you decide which method might be best for your lifestyle, preferences, and health needs. 

Take care, 

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