How the Pill Works

Ways Hormonal Contraception and the Pill Prevent Pregnancy

how the pill works
How the Pill Works. Nancy R. Cohen/Photodisc/Getty Images

Do you know how the pill works to prevent pregnancy? After all, the pill is one of the most popular of all hormonal methods. It is a very effective method of contraception. Ever wonder just how the pill works, and why it is so successful?

The Pill Shouldn't Get All the Attention!

In order to understand how the pill works, we need to back track, just a bit. To really understand how  "the pill" works to prevent you from getting pregnant, it is important to realize that this information applies to all types of hormonal contraception -- not just literally the birth control pill.

For the most part, all hormonal contraceptives work the same way to prevent pregnancy. It does not make a huge difference if the hormonal birth control method is a combination method (meaning it contains both estrogen and progestin) or if it is a progestin-only method.

So How Does the Pill Work?

There are three ways that the pill works to prevent pregnancy.

  1. The main way the pill works is by preventing a woman's body from ovulating during her monthly menstrual cycle. So, if the ovary does not release an egg, then there is nothing there for a sperm to fertilize. The pill (and hormonal contraception) may also make the fallopian tubes less likely to move an egg toward the uterus.
  1. The next way that hormonal contraception and the pill work is that they help to thicken your cervical mucus (the fluid found around the cervix/opening of the uterus). The pill makes your mucus sticky, so when sperm try to get through the cervix, this sticky mucus makes it much harder for them to swim through -- so it is more difficult to reach and fertilize the egg.
  1. The final way that the pill works to prevent pregnancy has to do with the uterine lining. Hormonal contraceptives may cause changes to the lining of the uterus. The hormones in these methods can thin out or prevent the growth of uterine tissue. This can lower the chances that implantation will take place.

Hormonal contraception and the pill can work in any or all of these ways to prevent pregnancy. To be most effective, these methods must be used consistently and correctly. Hormonal contraceptives (as a group) are between 92% to 99.9% effective. This means that of every 100 women who use hormonal contraception, 8 will become pregnant (with typical use) within the first year and less than 1 will become pregnant with perfect use (effectiveness rates vary slightly -- see: Effectiveness of Prescription Birth Control for the failure rates of each specific method).

Does the Pill Work During the Placebo Week?

This confuses a lot of women. First of all, what even is a placebo week? When is it? The placebo week is a time when your pill back contains "placebo" pills that contain no hormones or a small amount of hormones (less than the amount in the rest of the pack.

It can also be the time of your cycle when you are not using a specific method. Not all hormonal methods have a placebo week. So:

  • For of a typical 28-day pack of combination birth control pills, the fourth week is the placebo week.
  • For extended cycle pills with a 91-day pack like Seasonique and LoSeasonique, week 13 is the placebo week.
  • For a 21-day pack, like Loestrin, the fourth week is the placebo week (there are no pills to take this week).
  • For a 24/4-day pack, such as Yaz or Beyaz, the last four days are the placebo time.
  • For the NuvaRing, you take the NuvaRing out after week 3, and do nothing during week 4 (which is the placebo week).
  • For the Patch, you put on a new patch weeks 1, 2, and 3. You do not apply the patch week 4 (which is the placebo week).

All of these birth control methods can still prevent pregnancy during the placebo week/time. And you will most likely have your "period" (a withdrawal bleed) during this time.

So... yes -- the pill still works during this week, even though you are not taking any active hormones.  This means that you are still protected against getting pregnant if you choose to have sexual intercourse during the placebo week/time. You do not need to use a back-up birth control method if you have sex during this time.


Shoupe D, Mishell D. (2016). "The Handbook of Contraception: A Guide for Practical Management." (2nd ed). Totowa, NJ: Humana Press.

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