Final answer:
The risk of pregnancy is highest when unprotected sexual intercourse occurs on the day of or the day before ovulation. The fertile period extends from a few days before until a day after ovulation. Fertility awareness methods, which involve tracking the fertile window, have a typical-use failure rate of 20-25% due to variability in menstrual cycle lengths.
Step-by-step explanation:
The risk of pregnancy varies depending on when unprotected sexual intercourse (UPSI) occurs relative to ovulation. Ovulation is the release of an egg from the ovary and typically occurs around day 14 of a 28-day menstrual cycle. The egg remains viable for fertilization for about 24 hours. Sperm, on the other hand, can survive in the female reproductive tract for up to five days.
Unprotected sex on the day of ovulation carries the highest risk of pregnancy because the egg is immediately available for fertilization. Having UPSI the day before ovulation is also high risk, as sperm could survive until the egg is released. Generally, the fertile period in a cycle extends from a few days before ovulation to about a day after. During this time, the chance of pregnancy increases if UPSI takes place.
It should be noted that fertility awareness methods, which involve identifying the fertile window, have a failure rate of 20 to 25 percent under typical use. This is because cycle length and timing of ovulation can vary greatly among individuals. As such, sex near the anticipated day of ovulation could lead to pregnancy if predictions are inaccurate.