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Fern test refers to detection of a characteristic 'fern like' pattern of vaginal secretions when a specimen is allowed to dry on a glass slide and is viewed under a low-power microscope. The fern test is used to provide evidence of the presence of amniotic fluid and is used in obstetrics to detect preterm premature rupture of membranes and/or the onset of labor. It also may provide indirect evidence of ovulation and fertility, although it does not predict the time of ovulation.
Ferning occurs due to the presence of sodium chloride in mucus under estrogen effect. When high levels of estrogen are present, just before ovulation, the cervical mucus forms fern-like patterns due to crystallization of sodium chloride on mucus fibers. This pattern is known as arborization or 'ferning'.
When progesterone is the dominant hormone, as it would be in the second half of a normal cycle, the fern pattern is no longer discernible, and the pattern is completely absent by the 22nd day of a woman's cycle. The disappearance of the fern pattern after the 22nd day suggests ovulation, and its persistence throughout the menstrual cycle may suggest an anovulatory cycle.
- Patrick Duff, MD (2016). Preterm premature (prelabor) rupture of membranes. In Vanessa A Barss, MD, FACOG (Ed.), uptodate Retrieved from http://www.uptodate.com/'