When we talk about fertility we are talking about the combined fertility of the couple.
A woman is only fertile for 12 to 24 hours per cycle (the lifespan of her egg). But as sperm can live inside a woman’s body for up to 5 days, the number of potentially fertile days jumps up to about 6. (5 for the sperm, one more for the egg).
You are generally considered infertile for the first 3 days of your period, as long as you had a temperature shift 12 to 16 days prior to it.
You are generally considered infertile the evening of any day that you observed no cervical fluid. Once you start observing cervical fluid, you’re then considered fertile until you confirm ovulation with the two post-ovulatory rules.
You are generally considered infertile the evening of the 3rd day after your temperature shift.
You are generally considered infertile the evening of the 4th day after your most fertile CF, once your cervical fluid has become infertile again, and you have observed a clear temperature rise.