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What Is a Fern Test?

Mary McMahon
Updated Mar 03, 2024
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A fern test is an assessment of cervical mucus to determine if a patient’s amniotic membranes are leaking or to monitor fertility. The test involves taking a swab of mucus, allowing it to dry on a slide, and looking at it under a microscope to check for distinctive patterns of crystallization. If a leafy, fern-like pattern is present, it can be a useful diagnostic indicator; this pattern is created by sodium chloride and other minerals that can crystallize out as the mucus dries. These levels can fluctuate depending on hormones and whether amniotic fluid is present in the sample.

When estrogen levels are high, more sodium chloride tends to be present in cervical mucus, making the fern test one method that can be used to assess hormone levels. If estrogen levels remain consistently high with no progesterone, as in infertile patients, the test will be positive throughout the patient’s cycle. When test results change depending on timing, these indicate that the patient’s hormone levels are fluctuating. It is also possible to use blood tests to check hormone levels.

The more common use of the fern test is in pregnant patients when there is a concern that amniotic fluid may be leaking. Ruptures in the amniotic membrane can be a cause for concern, as they may put the patient and fetus at risk of infection and other complications. In some cases, it may be necessary to deliver immediately to safely remove the fetus, or to prepare for early delivery with steroid injections and other measures to promote fetal lung development and health.

When cervical mucus from a pregnant woman is tested, it should have minimal to no ferning, indicating that the sample just contains mucus. When leafy patterns appear, it indicates that some amniotic fluid is present in the sample. Other medical testing may be recommended to determine whether the pregnancy is threatened and decide how to proceed. If the patient is close to term, early delivery may be recommended to address a suspected rupture as promptly as possible.

Sample collection is very important with a fern test, as poor collection practices can cause a false positive or disrupt the crystallization. Clean sample collection tools and slides are necessary, and the lab also handles the sample with care to make sure it dries in a clean environment where it won’t be contaminated. If the result is positive, a repeat test may be performed to confirm, and the patient’s history and current symptoms may also be considered.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a The Health Board researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By bear78 — On Mar 21, 2013

I had a small scare with my first pregnancy. I leaked some fluid when I was seven months and was given a fern test. Thankfully, the test came back negative. It was not amniotic fluid.

By donasmrs — On Mar 21, 2013

Fern tests are also used in women who are not pregnant to predict their ovulation days. The device is even sold online for people to use at home.

My sister-in-law bought one to predict her ovulation so that she can increase her chances of conceiving. She and my brother have been trying to get pregnant for some time.

She said that it's very easy to use. The device comes with a whole set containing some glass slides and even a small microscope. If the fern pattern comes up on the glass, she knows that she's about to ovulate. It's kind of cool.

By stoneMason — On Mar 20, 2013

I understand the use of a fern test to check if a woman has leaking amniotic fluid. But why do they use this test to check hormone levels?

It's not going to show exact levels and a blood test will almost always be required afterward. So why not just do a blood test directly?

Blood tests are easy too and test results come back fairly quickly. I usually get mine the same day or the next day.

How long does it take to get the results of a fern test?

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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