How Does the Birth Control Pill Work? Understanding Oral Contraceptives

Birth control pills are one of the most common forms of contraception in the United States. But what exactly happens in the body when someone takes the pill every day?

There are several types of oral contraceptives, but they generally work by using hormones to interfere with the normal reproductive cycle and make pregnancy less likely.

How Does Pregnancy Normally Occur?

During a typical menstrual cycle, hormones signal one of the ovaries to release an egg. This is called ovulation. If sperm are present and fertilize the egg, the resulting embryo travels toward the uterus. Implantation occurs when the developing embryo attaches to the lining of the uterus. The birth control pill works by interfering with the processes necessary for pregnancy to occur.

One Way the Pill Works: Preventing Ovulation

For combined oral contraceptives, which contain estrogen and progestin, the primary mechanism is preventing ovulation. The hormones in the pill suppress the normal hormonal signals from the brain that would otherwise stimulate the ovaries and trigger the release of an egg.

The Pill Also Makes It Harder for Sperm to Reach an Egg

Hormonal contraceptives also change the mucus produced by the cervix. Normally, cervical mucus becomes thinner around ovulation, making it easier for sperm to travel through the cervix.

Progestin in birth control pills makes this mucus thicker, creating another barrier that makes it more difficult for sperm to enter the uterus and reach an egg. So even if ovulation were to occur, the altered cervical mucus provides another contraceptive effect.

What About Implantation?

Although the primary mechanism of oral contraceptives is to prevent ovulation, research has identified endometrial effects as a secondary mechanism. Some FDA-approved labeling for combined oral contraceptives has described changes to the endometrium, or uterine lining, that may reduce the likelihood of implantation if fertilization were to occur.

In simple terms, if conception was successful, birth control pills may prevent the fertilized embryo from attaching to the uterus, resulting in a miscarriage.

What Types of Birth Control Pills Are There?

There are two major categories of daily oral contraceptives:

Combined Oral Contraceptives

These contain estrogen and progestin. They are the most common type of birth control pill and work by preventing ovulation. They also thicken cervical mucus and cause changes to the uterine lining.

Common prescriptions include:

• Junel Fe

• Loestrin

• Microgestin

• Aurovela

• Larin

• Nortrel

• Yaz

• Yasmin

• Sprintec

• Ortho-Cyclen

• Levora

• Seasonique

• Sronyx

There are many generic versions and formulations, so brand names can change or become unavailable over time.

Progestin-Only Pills

These contain progestin without estrogen. They primarily work by thickening cervical mucus and, depending on the specific pill, may also suppress ovulation. Some progestin-only pills suppress ovulation much more consistently than others. They also cause changes to the uterine lining.

Examples include:

• Norethindrone 0.35 mg

• Slynd (drospirenone 4 mg)

• Opill (norgestrel 0.075 mg)

Opill is notable because it became the first daily oral contraceptive available over the counter in the United States; other oral contraceptives remain available by prescription.

What Are the Side Effects?

Many women tolerate birth control pills well, but side effects can occur.

Common side effects include:

• Headache

• Nausea

• Breast tenderness

• Spotting or breakthrough bleeding

• Changes in menstrual bleeding

• Abdominal discomfort

These side effects are often most noticeable when beginning the pill and may improve as the body adjusts to the hormones. Some women also experience changes in acne, menstrual cramps, or the regularity and amount of menstrual bleeding.

Are There Serious Risks?

Serious complications are uncommon, but they can occur. Because combined pills contain estrogen, they can increase the risk of blood clots. Rarely, serious complications such as stroke or heart attack can occur. They can also increase blood pressure in some women.

Certain women should not use estrogen-containing contraceptives because of medical conditions or risk factors.

Quick Facts

The birth control pill uses hormones to make pregnancy less likely through several effects:

1. Prevents ovulation: The ovaries are prevented from releasing an egg.

2. Thickens cervical mucus: Sperm have a harder time passing through the cervix.

3. Changes the uterine lining: The endometrium becomes thinner, which may can make implantation less likely if fertilization were to occur.

***Birth control pills do not protect against sexually transmitted infections.

Medical Sources

• American College of Obstetricians and Gynecologists (ACOG), Combined Hormonal Birth Control: Pill, Patch, and Ring (ACOG⁠)

• American College of Obstetricians and Gynecologists (ACOG), Progestin-Only Hormonal Birth Control (ACOG⁠)

• U.S. Food and Drug Administration (FDA), Birth Control (U.S. Food and Drug Administration⁠)

• Centers for Disease Control and Prevention (CDC), Combined Hormonal Contraceptives (CDC⁠)

• U.S. National Library of Medicine, Mechanism of Action of Hormonal Contraceptives (PubMed⁠)