Determining the Coverline


What is a coverline?

A coverline is a horizontal line drawn on your BBT chart to separate your pre- and post-ovulation temperatures. Keep in mind that this is really just the separation of the 2 phases your menstrual cycle: the follicular and luteal phases. While the line itself doesn’t have any physiological meaning, it is designed to help you easily see your temperature shift, along with some other helpful information about the health of your cycle.

How do you draw a coverline?

For some, drawing a coverline can get a little tricky or feel a bit cumbersome, but we’re going to break down how to do it step-by-step, according to the Fertility Awareness Method Rules first.

  1. As you’re charting, keep an eye out a pattern of temperature for 3 days that appears higher than the previous 6 days. Okay, you might be wondering what counts as a “higher?” Well, “higher” means anything at least two-tenths (0.2) of a degree Fahrenheit above the previous 6 days.  (although 0.4 is better).

  2. Once you’ve identified those 3 days, take note of  the previous 6 days.

  3. Next, find the highest temperature of those 6 days and then draw a line through one-tenth (0.1) degree Fahrenheit above that temperature. Here’s an example: if your highest temperature in those 6 days is 97.5 degrees F, then draw a line through 97.6 degrees F. (Remember, your line should be straight.) Voila! This is your coverline.

Ideally, the rest of the temperatures during your luteal phase will be above this line, but sometimes factors like hormonal fluctuations or imbalances that can cause dips.

The rules described above are based on the Fertility Awareness Method, and this works best for those with clear BBT patterns.

How does Glow draw a coverline?

Glow has a proprietary method in calculating your coverline that incorporates other signals for a more complex determination based on your body’s patterns.

In fact, Glow draws two types of coverlines. Glow draws a solid coverline when we have determined conclusively that you have ovulated by a temporal shift in your BBT, that fits several rather stringent rules. Glow draws a dotted coverline when not all our conditions have been met but a coverline is projected.

The dotted line will become a solid line if and when you have met several criteria:

  • Inputted temperatures for 9 consecutive days

  • Had a temporal shift that lasted for a minimum of 3 days

  • Had temporal shift that occurred in proximity of the Glow calculated fertile window.  Shifts that occur way earlier and way later may be discounted.

Reference Source