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An animation of an IUD-umbrella blocking rainfall.

Illustration by Marta Pucci

Reading time: 5 min

How do IUDs prevent pregnancy?

Top things to know:

• Depending on the type of IUD, pregnancy is prevented by one of the following: thickening of the cervical mucus, preventing sperm from passing into the uterus, decreasing chances of sperm survival, and changing the endometrium.

• IUDs work differently than other forms of birth control.

• Copper IUDs work by releasing copper ions into the uterus, making it inhospitable to sperm

What is the IUD?

The IUD is a small, plastic, T-shaped device that is inserted into the uterus to prevent pregnancy (2, 3, 4). (Fun fact: the first IUD was made out of silkworm gut (5).) The IUD is inserted into your uterus by a healthcare provider (4). IUDs work up to 3 to 10 years (4), They can be removed at any time if you want to become pregnant or just want to take it out (3, 2). 

IUDs are some of the most effective forms of birth control (6). Like most reproductive science, the technology behind IUDs is fairly new. 

Two types of IUDs are available in the United States, hormonal and copper or non-hormonal (4). They each work slightly differently based on their makeup. Let’s take a look at how they work to prevent pregnancy.

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How the hormonal IUD prevents pregnancy

The hormonal IUD contains contains a progestin called levonorgestrel. A small amount  of progestin, which is embedded within the vertical arm of the T, is released daily into the uterus (1).

Progestin helps to prevent pregnancy in three ways: 

  • It thickens the cervical mucus to the extent that sperm cannot penetrate in order to fertilize an egg (1).

  • It changes the normal contractions in the fallopian tubes, making it more difficult for an egg to travel to the uterus (7). 

  • When sperm enters the body of a person who uses a hormonal IUD, the sperms’ development can be negatively affected, or they might not survive at all (2).

How the copper IUD prevents pregnancy

In a copper IUD, the horizontal bar of the device’s T is wrapped in thin copper wire, and each bar of the T has a copper collar (7). The copper releases copper ions into the uterus. The ions change the fluids in the fallopian tube and uterus, making them inhospitable to sperm (1). Sperm entering these parts of the body won’t be viable and can’t fertilize an egg (3). 

Foreign body reactions: The IUD’s last line of defense

While the primary methods of action for IUDs are outlined above, both types of IUD also have a secondary line of defense known as a “foreign body reaction.” 

So, what exactly is a foreign body reaction? It may sound serious, but it’s actually a process that keeps bodies healthy. And in this case, it works to prevent pregnancy. When a foreign object is inserted into the human body, the body responds by causing inflammation in the area (1). (Think about getting a splinter stuck in your finger. Your finger swells as your body tries to evict the invading splinter. Thanks, body!)

The body sends white blood cells into the uterus to prevent infection from the IUD (8).

The result is inflammation. Theoretically, this inflammation inside the uterus makes it difficult for an embryo to implant inside the lining of the uterus (8). However, this serves as a last possible line of defense against pregnancy, because IUDs greatly reduce the risk for fertilization in the first place by preventing sperm from reaching an egg as well as making cellular changes to the egg the fertilization process (8). 

Further, there is evidence that even if an egg is fertilized, it doesn’t undergo normal developmental processes and thus won’t develop into an embryo (8).

Do I ovulate while using the hormonal IUD?

Many people who use a hormonal IUD will continue to ovulate. But since the device contains a progestin that disrupts the hormone cycle (2), there is a possibility the device can impact ovulation, depending on its dosage. 

Hormonal IUDs are available with different doses of hormones. The highest dose impacts ovulation more than the lower doses (2, 9, 11). The effectiveness of an IUD is not impacted by whether or not a user ovulates.

A person using a 52 mg IUD may stop ovulating after the device is implanted. Ovulation can return as the hormonal potency wains (2).

  • In the first year, about 45 percent of people using a 52 mg IUD will ovulate. 

  • After four years, about 75 percent of people using a 52 mg IUD will ovulate (2). 

Most people using the 19.5 mg IUD continue to ovulate (10). In studies, people who use this dosage have normal ovarian function (10). The same is true with the 13.5 mg IUD (11).

As long as you replace your IUD on time, changes in ovulation will not impact the risk for pregnancy, because the IUD primarily works through the processes we listed above (2, 3).

Do I ovulate while using the copper IUD?

Some forms of birth control, like the pill, work by inhibiting ovulation. This isn’t the case with the copper IUD. Copper IUDs don’t impact ovulation at all. Menstrual bleeding may increase for the first three to six months, but bleeding should decrease over time (9). 

Learn more about period changes with the IUD

After two years, about half of people using the 52 mg hormonal IUD will stop experiencing periods (9). Lower doses of hormones impact monthly bleeding less (10, 11). If you are using the IUD, it’s helpful to track your bleeding patterns so you can evaluate how it’s working for you. Read more about bleeding on the IUD here.

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