A fern test, also known as amniotic fluid crystallization, is a test that is performed to determine the presence of amniotic fluid to help detect premature rupture of membranes. Close to term, amniotic fluid shows a typical fern pattern similar to that seen in cervical mucus; this occurs due to the presence of sodium chloride I mucus under a predominantly estrogen effect rather than progesterone.

This study differentiates urine from amniotic fluid. It is done to determine whether the fluid passed is urine or prematurely leaked amniotic fluid. Urine can be differentiated from amniotic fluid if the fluid is tested for the presence of urea, nitrogen, potassium, and creatinine and the absence of AFP. This is a relatively fast and inexpensive test that can be easily done.

1. Perform a vaginal examination with the use of a sterile speculum.

2. Swab the vaginal wall and then place a few drops of vaginal fluid on a slide and allow it to dry.

3. Look for a fern or palm leaf pattern (arborization) under the microscope.

1. A positive test indicates PROM or ruptured membranes at onset of labor.

2. A negative test shows no ferning or crystallization; this indicates little or no estrogen effect.

3. No fern pattern is seen if the specimen is urine.

Pretest Patient Care

1. Explain test purpose and procedure.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Blood contaminating the specimen inhibits fern formation.

Normal

• Positive: Ferning pattern on specimen glass slide confirms presence of amniotic fluid