This is one of the most common myths perpetuated by the medical community. In reality, the exact day of ovulation cannot be determined by the basal body temperature. Only about 10% of women even have a drop in basal temperature. And once the temperature has risen, it is virtually certain that the egg is already gone (assuming conception hadn’t occurred beforehand.) This is because an egg only lives 12-24 hours, and by the time the temperature has risen, the egg is no longer viable. Therefore, if a couple wants to achieve a pregnancy, the sign to focus on is not the basal temperature, but the cervical fluid.