Final answer:
The first fertile day for contraception using BBT cannot be determined through temperature alone, as the rise in BBT only signals that ovulation has already occurred, and the fertile window may have started days prior.
Step-by-step explanation:
The first fertile day for contraception using basal body temperature (BBT) is not the day of the temperature spike but rather a few days before ovulation. BBT tracking is not an effective contraceptive method on its own because it can only signal after ovulation has already occurred. Considering that sperm can live for up to a week in the female reproductive tract, by the time the temperature rise is observed, the most fertile period may have already started or passed. Slight increases in BBT indicate that ovulation has occurred, and this rise generally means that the fertile window was already present a few days before. It is important to combine BBT monitoring with other signs such as changes in cervical mucus or use ovulation prediction kits for more accurate fertility awareness.