Your body temperature rises and falls in a consistent pattern. Ovulation causes your body temperature to remain elevated for about two weeks. GenderLabs™ looks for this rise to determine when post-ovulation infertility begins. 

Body temperature or more accurately, Basal Body Temperature (BBT) is another one of the body's measurements that changes in regular cycles. "Basal" body temperature simply means the temperature of your body at rest. Your body temperature is not a consistent 98.6° F / 37° C as most people think. These figures are the body's normal base high temperature during waking hours. After going to sleep at night, your body temperature gradually drops, reaching its lowest point in the early morning. As you get closer to your waking hours, your metabolism picks up again, raising your body temperature ever so gradually, and the cycle is repeated. The scale of the drop and rise of body temperature differs from person to person, but all people experience it to some degree or another (no pun intended).

When a woman ovulates, the shell containing the ovum ruptures, releasing the egg and the hormone progesterone into the body. This hormone has several effects, for our purposes not the least of which is the raising of the basal body temperature. This increase in temperature is significant and detectable, as it typically is about 0.4° Fahrenheit / 0.2° Celsius over the course of several days. To detect this change, you will need a basal thermometer, available from most pharmacies. A standard fever thermometer is not accurate enough for our purposes here, as even a change of 0.1° F / 0.05° C is important. Therefore, an important first step in the practice of FAM is to buy a Basal Thermometer. GenderLabs™ looks for three temperatures that rise at least 0.2° F / 0.1° C above the high of the previous six temperatures. A "trendline" or "coverline" as referred by some quarters, is drawn 0.1° F / 0.05° C above the highest of these six temperatures and when three successive temperatures are above this line, a thermal shift is identified and there's a good chance that ovulation has already occurred.