The life span of a fertile egg is about 24 hours and that of sperm is about three to five days. If a woman engages in unprotected sex about five days to ovulation, she can get pregnant.
If a woman has 28 days cycle, she should not have unprotected sex from 9th day, i.e., five days to the 14th day of the cycle. She should also not do so on the day of ovulation, 14th day and at least 48hours after ovulation. This period of possible pregnancy is called window period.
This calculation in relation to pregnancy is only useful for those who have consistent/regular menstrual cycle. It cannot be relied upon by women who have irregular menstrual cycle. For example, her cycle can last for 27 days this month and then to 29 days in the next etc. Others too have consistent cycle, say, 29 days in every month for a long time.
Also, click links below to read more on my posts on this subject for clearer understanding
Common Menstrual Problems & What to Do
How to Calculate Total Days in Your Menses Cycle-No Phone Apps
