To calculate the time of the next menstrual period, a cycle is counted from the first day of the first menstrual period to the first day of the next menstrual period. Some people calculate the menstrual cycle from the clean start, this is not correct, so the vast majority of people’s menstrual cycle should be 28-30 days once. Because of individual differences, everyone has their own menstrual cycle, as long as the menstrual cycle is not less than 25 days, and regular, are considered normal. Some people have their periods only twice a year and can get pregnant and have children normally. Some people have their periods once a month, while others have their periods once every two months, so the next period is calculated from the first day of the current period to the first day of the next period as a cycle.