You can download a free full version of LifeCycle fertility tracking software to help you chart and interpret your btt and signs of ovulation or a free BBT charting tool for your spreadsheet program. Only by review of previous months BBT experience can you speculate ovulation for the current month. The progesterone stays in your system for 11-14 days and then drops if the egg has not been fertilized. If an egg is fertilized however, the BBT will remain high as your body continues to produce progesterone. If you have high temperatures for 18 consecutive days, you can be pretty sure that you are pregnant.
Some individuals show a third rise in BBT if an egg is fertilized.
This third rise occurs about 6-8 days following ovulation. This is when a fertilized egg implants into the uterine lining which causes another surge of progesterone. The appearance and consistency of the cervical fluid changes throughout the stages of the menstrual cycle. Observation of these changes can be another excellent way to predict your fertile times. It is best to check your cervical fluids mid-day, or at a time when you are not particularly tired, because the cervix tends to be lower and drier when you are tired.