How birth control pills work by tricking the body into thinking it's already pregnant

How birth control pills work by tricking the body into thinking it's already pregnant
Birth control can come with side effects like nausea and headaches.Sarah Schmalbruch/INSIDER
  • The pill is one of the most popular types of birth control.
  • Birth control pills work by tricking the body into thinking it's pregnant, which halts ovulation.
  • The pill is 99% effective is taking correctly, but due to human error, typical use makes it more like 91% to 93% effective.

One of the most popular birth control methods is the pill. From 2015-2017, 12.6% of US women using contraception were taking the pill compared to just 8.7% who used condoms.

The pill is a type of hormonal birth control that contains both synthetic versions of estrogen and progestin. These hormones disrupt the body's natural cycle, which prevents pregnancy.

However, the pill can come with side effects and is not always 99% effective at preventing pregnancy. Here's what you need to know when considering oral contraception.

The pill can be 99% effective

The birth control pill is 99% effective when used correctly. Correct use means taking the pill at the same time every day and never missing a dose. However, this can be difficult if you have a hectic schedule like when traveling.

There are also other factors that can reduce the pill's effectiveness like vomiting right after taking it or certain prescription medications like:

  • Antibiotics
  • Antifungals
  • HIV medicines
If your doctor prescribes you a medication, ask them if it will interfere with your birth control. If the answer is yes, use a back-up method during sex, like condoms or spermicides.

Accounting for human error, typical use of the pill makes it 91% to 93% effective at preventing pregnancy — meaning around 7 out of ever 100 people who use it will get pregnant.

More effective methods of birth control include IUDs and implants, which are 99% effective, or the birth control shot which is 96% effective, according to the Centers for Disease Control and Prevention.

3 ways the pill prevents pregnancy

Traditional hormonal birth control pills contain artificial versions of the sex hormones estrogen and progestin. (Though you can get progestin-only minipills.)

The body naturally produces versions of these two hormones — called estrogen and progesterone — which help regulate a woman's menstrual cycle and ability to get pregnant.

The birth control pill works by essentially tricking the body into thinking it's pregnant. When a sperm fertilizes an egg, the woman's estrogen and progesterone levels change to prevent future ovulation. Birth control pills work in the same way.

No ovulation means no egg to fertilize and no accidental pregnancy. However, sometimes an egg can still develop and reach the uterus, especially if you miss a dose. Birth control can still prevent fertilization in two ways:
  1. Thickening of the cervix. The cervix separates the vagina from the uterus and is a pathway for sperm to reach the uterus where it can fertilize an egg. So by thickening the mucus in the cervix, the pill's hormones make it more difficult for sperm to reach the uterus in the first place.
  2. Thinning of the uterine lining. After an egg is fertilized, the next step in pregnancy is implantation, where the egg attaches to the uterine wall. However, if the uterine lining is too thin, the egg won't attach and the pregnancy will not progress.

Types of birth control pills

A common type of birth control pill is one that comes in a 21-day pack. So you take one pill every day for three weeks straight and then have a one-week break where you get your period.

However, there are other options, according to The American College of Obstetrics and Gynecology (ACOG), including packs of 28-days, 90-days, and 365-days.

The main difference between types is how often you get your period. Moreover, studies show that larger packs are equally effective as 21-day or 28-day packs.

Birth control pills take 7 days to work

You can start taking birth control pills as soon as you get them. It doesn't matter if you're on your period, or not. Just find a time of day where you can remember to take them regularly.

A combination pill containing both estrogen and progestin will start to become effective seven days after starting the pill. For progestin-only minipills, they can take effect after just 48 hours. Therefore, make sure to use a back-up method until the pill kicks in.

There's a workaround to make combination pills work slightly faster. If you start the pill within 5 days after the start of your period, you're protected immediately.

In general, after stopping any form of contraception, it is a good idea to wait for your first normal period before you start trying to conceive.

Side effects of the pill

Sometimes the body can react adversely to the hormones in the pill, which can lead to side effects. The most common side effect is bleeding between periods, also called breakthrough bleeding. Other side effects include:
  • *Nausea
  • Headaches
  • Cramping
  • Breast tenderness
  • Vaginal discharge
  • Decreased sex drive

*You can reduce or prevent nausea by taking the pill with food or right before bed.

Weight gain is often misrepresented as a common side effect of birth control. But this is not the case. The only form of birth control proven to lead to weight gain is a progestin-only hormonal injection called Depo-Provera.

Not everyone experiences side effects. Moreover, as the body adjusts to the hormones, any uncomfortable side effects should resolve within 2-3 months of starting the pill. If you're still experiencing side effects after a few months, discuss with your gynecologist about changing to a different pill.

Other side effects from the pill can be beneficial, like helping reduce acne or alleviating painful period cramps.

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