From FF's emails:
The primary fertility signs are your basal body temperature which rises after ovulation, and your cervical fluid which gets increasingly wet, clear and stretchy as ovulation approaches.
The days when conception is possible are the days just before and including the day of ovulation. Based on the MAXIMUM lifespan of human sperm and ova, (five days for sperm and one day for the ovum) this fertile window is at MOST six days. This MAXIMUM window is thus made up of the five days before ovulation and the day of ovulation.
Though pregnancy is technically possible during this six day window, most pregnancies actually can be attributed to intercourse from within TWO DAYS before ovulation up to the day of ovulation.
This makes for a practical fertile window of just THREE DAYS each cycle: the TWO DAYS before ovulation and the DAY OF OVULATION itself.
Charting your fertility signs helps you to find those days so that you can best time intercourse to get pregnant. Couples who are aware of their fertile time and focus intercourse during this time have been found to have MUCH HIGHER CONCEPTION RATES than couples who are unaware of their most fertile time.
You may have been told that you ovulate or are most fertile on day 14 of your menstrual cycle, mid-cycle, or even 14 days before your period is expected. These assumptions are based on broad averages and they may not be true for you, even if your cycles are regular.
Your own fertility pattern is unique to you and charting your fertility will help you to see it and find the days that you are most likely to conceive.
To maximize your conception chances, it is recommended to have intercourse every day or AT LEAST every other day during your fertile time until a clear and sustained thermal shift can be detected on your chart.