No one can be 100 percent sure. But chance is negligible.

Menstrual cycle is calculated from the first day of your period to the first day of the next period . Safe period is from day 1 to 7 and day 21 to the rest of the cycle, when you can have sex without worrying about getting pregnant. This period will not be the same for all women as it depends on the length of the menstrual cycle. The average menstrual cycle is 28 days or a little less, but if your cycle is longer than 28 days, the safe period is not reliable.