First period symptoms: Three signs that your period is coming
A typical menstrual cycle is 28 days. This means that most people will get their period about every 28 days. In reality, that 28 number actually varies from girl to girl. A normal cycle can be anywhere between 21 and 35 days.
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Start by recording in your diary, or phone, the day your period started each month. Do this for a few months. Once you have a few months recorded, add up the number of days in each cycle (i.e. the number of days between the first day of one period and the day before your next period). Divide this number by the number of cycles. This gives you the average number of days in your menstrual cycle.