What is Coverline?
When trying to get pregnant, it is a good idea to chart your basal body temperature to determine when and if you ovulated. Ovulation is associated with an increase in temperature -- a thermal shift -- revealing a typical biphasic curve. The cover line is a horizontal line drawn after ovulation to help differentiate temperatures before ovulation (low) and temperatures after ovulation (high). When your waking body temperature rises more than two-tenths of a degree higher than the previous six days, and it stays that way for at least two days, your cover line can be established. When your waking body temperature rises more than 0.2 or two-tenths of a degree higher than the previous six days, and it stays that way for at least two days, you can start drawing your cover line. This rise normally occurs during your thermal shift the day after ovulation. For example, a jump from 97.1 to 97.3 is a rise that is 0.2 or two-tenths of a degree higher.
The cover line is drawn after your temperature has risen because of ovulation and stayed up for at least 2-3 days. This is called a "sustained" elevation and indicates that ovulation has taken place. It is possible that prior to the rise, prior to ovulation, the temperature may have dropped for a day below what eventually will be the cover line. That one-day temperature should be ignored in order to create the cover line. Understanding how to chart your basal body temperature and establish a cover line is a useful skill in fertility charting.