By your late teens, most people's periods have settled into a pattern and come roughly every three to five weeks. A cycle that is a few days shorter or longer than the last one is still normal.
Your cycle is counted from the first day of one period to the first day of the next. So cycle length means the number of days between one period starting and the next one starting. It isn't how many days you bleed for.
What happens in a cycle?
Your brain and your ovaries talk to each other using hormones, which are chemical messengers carried in your blood. Two of the main ones are oestrogen and progesterone. In the first part of the cycle, oestrogen builds up the lining of your womb. Around the middle, one of your ovaries releases an egg. This is called ovulation. After that, progesterone keeps the lining in place. If the egg isn't fertilised, progesterone drops and the lining comes away. That is your period.
Why was it unpredictable at first?
Most people get their first period between the ages of 12 and 13. For the first year or so, the body is still learning this routine, and many cycles happen without ovulation. That's why early periods can be hard to predict, and some months may be skipped. The international guideline on PMOS (formerly known as polycystic ovary syndrome PCOS, a common hormone condition) says irregular cycles in the first year after your first period are a normal part of puberty.
What counts as regular now
Doctors look at how many years have passed since your first period:
One to three years after your first period, cycles between 21 and 45 days are within the normal range.
More than three years after, cycles are expected to be between 21 and 35 days, with at least eight periods a year.
If you're 16 to 20, you're most likely in the second group. Small changes from month to month are still normal. Stress can delay a period and so can being ill.
If you take the combined pill, the bleed in your break week happens because you stop taking the hormones for a few days. It's called a withdrawal bleed. It doesn't tell you much about your own natural cycle, so a regular bleed on the pill can hide a cycle that would be irregular without it.
When to see a doctor
More than a year after your first period, it's worth getting checked if:
you go more than 90 days without a period, even once
your cycles are often shorter than 21 days
more than three years in, your cycles are often longer than 35 days, or you have fewer than eight periods a year
you haven't had a first period by the age of 15
Irregular periods have several possible causes. PCOS/PMOS is a common one. Others include a thyroid problem (the thyroid is a small gland in your neck that controls how fast your body uses energy) and not eating enough for the amount of exercise you do. A doctor can usually start with a few questions and a blood test.
Tracking your cycle for three months makes any pattern much easier to spot. Our free printable cycle tracker is a simple way to do this and you can take it to your appointment.