Ovulation refers to the rupture of the ripened follicle in the ovary to discharge the egg ready for fertilization at middle of the menstrual cycle. For example in a 28-day cycle, ovulation occurs on the 14th day. Variations in the timing are not uncommon and it can range from the ninth day to the 16th day before the next menstrual period. So the chances of fertilization are more few days around the ovulation time. That’s why the time around mid cycle is considered unsafe.
An ovum can survive for about 12 hours after it leaves the ovary. It can get fertilized only if it comes into contact with a sperm within this time frame. Sperms, however, can live in the female genital tract for 48 hours or even 72 hours. So if a woman with a 28-day cycle wants to conceive, she should try between day 11 and day 14.
Any alteration of the menstrual cycle from the standard 28-days cycle will also alter the ovulation time and safe period.
The so-called safe period or the rhythm method of contraception is safe if the chances of intercourse happen in the first eight days or last 10 days of a menstrual cycle. For all practical purposes, the safe period is not totally safe as the time of ovulation varies from cycle to cycle and from individual to individual.