The ovulation in women happens exactly fourteen days before the end of the menstrual cycle. So, if the menstrual cycle is with regular length it is easy to assume when the ovulation might occur. (Example: length of menstrual cycle=30 days; ovulation occurs at 16th day). Because you can never be sure how long the menstrual cycle will, avoiding sexual intercourse at least five days before and after the calculated ovulation date decreases significantly the possibility for pregnancy. However, this is not considered 100% safe method to avoid unwanted pregnancy.