The menstrual cycle begins on the first day of your period and ends the day before the beginning of your following period.
A regular or irregular cycle
A “regular” menstrual cycle is one that always lasts the same amount of time. The average length of a menstrual cycle is 28 days, but it can range anywhere from 21 to 35 days. In the first few years of having your period, it is normal for them to be irregular. You might therefore have a longer or shorter cycle; it can even change from one cycle to the next.
Can it change?
Yes! Your cycle can become irregular if you are going through something difficult or demanding, or because of stress or fatigue. Training for a high level sport and nutrition can also affect your cycle.
When do I ovulate?
You ovulate 14 days before the end of your cycle (before your next period). Your “fertile window” (the time in your cycle when there is a risk of getting pregnant if you have unprotected penis-to-vagina sex) begins 5 days before you ovulate and ends 3 days after.
Important! This means that if your cycle is irregular (often the case during adolescence), it is impossible to know when you will ovulate based solely on the calendar. In fact, you will only be able to calculate when you ovulated after the fact when your cycle ends (by subtracting 14 days from the day before your period started).
On top of all that, even if you do have a regular cycle, remember that many factors can affect the length of your cycle (like stress for example). This is why it isn’t recommended to rely solely on the calendar to assess when you are at risk of getting pregnant. Instead, find out about other types of contraception (hormonal or not).