While the science behind this ferning phenomenon is several decades old, it was 2001 before the first optical viewer was approved in the United States. Similar devices have been in use in Europe for many years.

Several different brands of scopes are available: some are smaller and more convenient than others, some allow a woman to track more than one day's results, some resemble miniature microscopes and others are shaped like a lipstick.

They all work virtually the same - the woman takes a small sample of either her saliva and puts it on either a slide or directly on the scope's viewer, waits a few seconds or minutes for the sample to dry, and views the sample through a magnifying scope. When the woman sees fern-like structures formed by the crystals in her sample, she is assumed to be near ovulation.

While ferning-scopes can help a woman predict her most fertile time, reading the results correctly can be challenging. There are some non-ferning crystal formations that may be viewed during other, less fertile times of a woman's cycle. Users of these devices will need to become very familiar with the specific type of crystal formations that are known as the 'fern effect.'

In addition to information provided by the manufacturer, there are a number of websites with explicit and helpful instructions on reading ferning-scopes.

One of the benefits of ferning scopes is that they are re-usable, so that consumers can re-test with the same scope through many reproductive cycles.