How the Pill Works to Prevent Pregnancy

Have you ever wondered how the pill works to prevent pregnancy? After all, the pill is one of the most effective forms of contraception, with 41% of women between the ages of 15 and 24 and 22% of women between 25 and 34 using it, according to the Kaiser Family Foundation.

Here are some facts that can help you decide if the pill is the right option for you.

birth control pills
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About Hormonal Contraceptives

For the most part, all hormonal contraceptives—including the pill—work the same way to prevent pregnancy. It doesn't make a difference whether the hormonal birth control method is a combination drug containing both estrogen and progestin or if it is a progestin-only option.

All hormonal contraceptives contain man-made hormones that mimic those that direct the menstrual cycle. Taking them on an ongoing basis inhibits the cycle and prevents pregnancy from occurring.

Using estrogen and progestin together prevents ovulation while effecting changes in the uterus and cervical mucus that make pregnancy near-impossible. By comparison, progestin-only contraceptives like Depo Provera, Nexplanon, and Mirena IUD prevent ovulation only (and with varying degrees of success).

To be most effective, hormonal contraceptives must be used consistently and correctly. As a group, they are between 92% and 99.9% effective. This means that of every 100 women who use hormonal contraception, eight will become pregnant with typical use while roughly one in a thousand will become pregnant with perfect use.

How the Pill Works

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


The main way the pill works is by preventing a woman from ovulating during her monthly cycle. So, if the ovary does not release an egg, then there is nothing there for a sperm to fertilize. The pill may also make the fallopian tubes less likely to move an egg toward the uterus.

By inhibiting hormonal fluctuations during the menstrual cycle, the pill prevents follicle-stimulating hormone (FSH) levels from rising. Without ample FSH, ovulation cannot occur.

Cervical Mucus

The next way that the pill prevents pregnancy is by thickening your cervical mucus (the viscous fluid found around the opening of the uterus). The pill makes your mucus stickier than normal, reducing the ability of sperm to pass through the cervix and reach the egg.

Under normal circumstances, the mucus will thin when ovulation occurs, which not only eases the passages of sperm but provides them a hospitable environment.


The final way that the pill works to prevent pregnancy involves the uterine lining (called the endometrium). Hormonal contraceptives thin the endometrium and make it far more difficult for a fertilized egg to implant. Without implantation, the egg cannot receive nutrients and blood needed to survive.

Under normal circumstance, the endometrium will thicken during ovulation to create a place where a fertilized egg can implant and grow.

Placebo Week

Placebo week is a time when your pill back contains placebo pills ("sugar pills") with either no hormones or reduced hormones. In effect, it is the time in your cycle when you are not taking any pills. Not all hormonal methods have a placebo week.

The placebo week can vary based on the contraceptive types:

  • For a typical 28-day pack of combination birth control pills, the fourth week is the placebo week.
  • For extended cycle pills (continuous birth control) 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 Ortho-Evra Patch, you put on a new patch during weeks 1, 2, and 3. You do not apply the patch week 4 (which is the placebo week).

The placebo week is important in that it helps you maintain a consistent habit of taking a pill every day, rather than stopping and starting. Moreover, you are likely to have your period during this week, making pregnancy all but unlikely.

Even though you are being exposed to few, if any, hormones during the placebo week, the level of protection remains unchanged. As such, you do not need additions forms of contraception to prevent pregnancy.

A Word From Verywell

The bottom line is that the pill, as well as other forms of hormonal contraception work in more than one way. They can prevent a woman from ovulating, they can lead to a thicker cervical mucus which hinders the passage of sperm through the cervix, and they can change the lining of the uterus in a way which inhibits implantation if fertilization occurs.

The pill continues to work through the month, even when you are taking placebo pills so that additional forms of contraception aren't needed.

With that said, the pill does not reduce the risk of sexually transmitted diseases, including HIV. For this, condoms still offer you the best means of protection.

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Article Sources
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  1. The Kaiser Family Foundation. Oral Contraceptive Pills. Published May 23, 2019.

  2. The Department of Health of Manila. The Philippine Clinical Standards Manual on Family Planning 2014 Edition. 2014.

Additional Reading
  • Kaiser Family Foundation. Oral Contraceptive Pills. San Francisco, California; May 23, 2019.

  • Cunningham, F. Gary., and John Whitridge Williams. Williams Obstetrics. New York: McGraw-Hill Education Medical, 2014. Print.