According to BabyMed, the basal body temperature of a woman has a strong likelihood of being elevated 14 to 15 days after ovulation if she is pregnant. This is particularly the case if she has not had her menstrual period or missed her period. Prior to those 15 days, it is nearly impossible to use basal body temperature as an indicator of pregnancy.
BabyCenter defines basal body temperature as the lowest temperature in the human body within a 24-hour period. It is a good idea for women to obtain their BBT in the morning at the same time each day to notice ovulation patterns. Prior to ovulation, the average temperature is 97.2 to 97.7 degrees Fahrenheit. Two to three days prior to ovulation, the BBT increases 0.4 to 1.0 degree. This lets a woman know she is likely going to ovulate in the next few days, but it takes a few months of charting her basal body temperature to notice patterns well enough to ensure she is ovulating. This increased temperature generally lasts until the next menstrual period occurs. According to BabyCenter, if the temperature does not go back down and a woman has missed her period, pregnancy is probable.