Considering the cycle duration of 28 days and cycles being regular 10 to 16 days of cycle(starting day of period is counted as day 1)are considered as unsafe as ovulation occurs during that time and if ova is fertilized pregnancy occurs.But sperms can survive in the female genital tract for about 72 hours so fertilization can occur after that also.
It is not a very reliable method of contraception so any other other form of contraceptive should be used.
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