What Is a Fern Test?

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.