Pregnant people make high levels of estriol, an estrogen hormone. This hormone helps their uterus grow and stay healthy. It also prepares as their body for childbirth and breastfeeding. Estriol levels that are too high or low may indicate a problem with the baby or pregnancy. Providers may also use estriol as a hormone therapy to ease menopause symptoms.

What is estriol?

Estriol is one of three estrogen hormones. Estriol levels rise throughout pregnancy, helping to keep your uterus and unborn baby healthy. The levels are at their highest right before childbirth. They help prepare your body for labor and delivery.

Everyone makes estriol. But in people who aren’t pregnant, the levels are almost undetectable. Other names for estriol include oestriol and E3.

What is estrogen?

Estrogen is one of the main female sex hormones. Men and people designated male at birth also have estrogen, but in much lower amounts than people designated female at birth.

This hormone plays a critical role during puberty in female sex traits like developing breasts and pubic hair. Estrogen is also important for menstruation and pregnancy. The ovaries (part of the female reproductive system) produce most of your estrogen.

What are the types of estrogen?

There are three main types of estrogen:

  • Estrone (E1): People make this type of estrogen after menopause.
  • Estradiol (E2): This is the most common type of estrogen that people make.
  • Estriol (E3): This is the main type of estrogen in a pregnant person’s bloodstream.


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How does your body make estriol?

Your developing baby’s adrenal glands make a hormone (a type of chemical) called dehydroepiandrosterone sulfate (DHEAS). The baby’s liver changes this hormone into another chemical called 16a-hydroxy-DHEAS. This chemical travels to the placenta, where it changes into estriol. It then enters the pregnant person’s bloodstream, causing estriol levels to rise.

The placenta is a unique organ that only exists inside your uterus during pregnancy. Your unborn baby’s umbilical cord connects to the placenta, which attaches to your uterine wall. The placenta functions similarly to your endocrine system. It provides nutrients, hormones and oxygen to your unborn baby. It also helps remove waste products from the fetus’s blood.

What does estriol do in your body?

During pregnancy, estriol:

  • Helps your uterus grow as the fetus gets bigger.
  • Makes your body more sensitive to other pregnancy hormones.
  • Prepares your body for labor and delivery and breastfeeding.


What do changes in estriol levels mean?

Estriol levels start to rise around the eighth week of pregnancy. They continue to increase throughout pregnancy. Estriol levels are at their highest about three weeks before you give birth. You may go into labor when estriol becomes the dominant pregnancy hormone.

During pregnancy:

  • Low levels of estriol may show a problem with the placenta. This result can also signal a problem with your baby’s health, like Down syndrome.
  • A sudden increase of estriol before the 37th week of pregnancy may mean you’re at risk for premature labor. Healthcare providers don’t check estriol levels for this reason.

People who have low levels of estriol right before childbirth may not go into labor on their own. Healthcare providers will use labor induction methods to start labor artificially.

How do healthcare providers measure estriol levels?

A blood test or urinalysis measures estrogen levels, including estriol levels during pregnancy. A blood test is a simple blood draw in your healthcare provider’s office or a lab.

A 24-hour urine sample test is the most accurate way to measure estriol levels. You collect urine in a container over 24 hours, refrigerating the container between uses. After 24 hours, you return the filled container to your provider’s office or lab.

There are also at-home test kits that measure estrogen in saliva. But if you’re pregnant, your provider will want to use a more reliable method.


When do healthcare providers measure estriol levels?

Providers typically test estriol levels between the 15th and 20th week of pregnancy, during the second trimester. This prenatal test is part of a triple marker or quad marker screening that helps detect birth defects.

A quad marker screening test measures the hormones estriol and human chorionic gonadotropin (HCG). It also measures alpha-fetoprotein and inhibin-A proteins.

Your provider may order an estriol blood test or quad screen if you have factors that increase the risk of birth defects, such as:

What is the role of estriol in hormone therapy?

Healthcare providers may prescribe laboratory-made estriol (a bioidentical hormone) as part of hormone therapy for menopause. Bioidentical hormones mimic your body’s natural hormones. You may take oral medication, apply a topical cream to the skin or insert a suppository into your vagina every day.

Because the U.S. Food and Drug Administration (FDA) hasn’t approved estriol for use in people, a specialized compounding pharmacy must make these customized drugs. The compounding pharmacy mixes estriol with other medications or a lab-made estradiol hormone based on your provider’s specifications.

Hormone therapy for menopause is an off-label use of estriol. Estradiol is the only FDA-approved bioidentical hormone for the treatment of estrogen-related problems. Off-label use is using an approved medicine for a different purpose.

As part of HRT, estriol may:

What are the side effects of estriol?

Estriol is the weakest of the three estrogen types. It's less likely to cause side effects like high blood pressure and blood clots than estradiol, the strongest estrogen.

Potential side effects of estriol include:

Is estriol safer than estradiol?

Because estriol isn’t as potent as estradiol, it may be safer. However, any type of estrogen therapy may increase your risk of:

What else does estriol do?

Researchers are still exploring the many functions of estriol. Lab-made estriol may also:

A note from Cleveland Clinic

During pregnancy, you make high levels of estriol, a type of estrogen hormone. This hormone keeps you uterus healthy and helps it grow along with the developing baby. Estriol levels peak toward the end of pregnancy to get your body ready for childbirth and breastfeeding. Changes in estriol levels may indicate a problem with the baby or pregnancy. Healthcare providers sometimes prescribe laboratory-made estriol as part of hormone therapy to ease menopause symptoms.

Medically Reviewed

Last reviewed on 02/18/2022.

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