A typical bbt spike will likely be between 0.4 °F and a full degree (or even a bit more). The minimum basal temperature increase could be 0.2°F. To draw a coverline on your BBT chart, you should follow the following steps
- You identify three continuous daily basal temperatures that are at least 0.2 or two-tenths of a degree higher than the previous six days. (6 Low and 3 high principle )
- Highlight those previous six days
- Find the highest of these six highlighted temperatures
- Draw a straight line that is 0.1 or one-tenth of a degree higher than that of the highest of the highlighted temperatures
- This line is your coverline
- The coverline should run straight horizontally across your chart
Here is an example of coverline on BBT charting. The first identified temperature above coverline is your BBT spike.
While the coverline is pretty useful when you use bbt charting to track your ovulation cycle, it is a time-consuming task to track and draw every cycle. Good news is that some of smart APPs like Premom integrate with the smart basal thermometer and support drawing the coverline automatically for you so that you are able to see the biphasic BBT pattern easily.