Periods and cycles · For ages 13–17
Why does my period come at different times?
It is common for the number of days between periods to vary, especially during the first few years after your first period.
What happens?
The time between periods can vary
A menstrual cycle is counted from the first day of one period to the first day of the next. It is regulated partly by hormones and does not have to be exactly the same each month.
During adolescence, the time between periods often varies because the hormonal system is still developing. Stress, illness, substantial changes in exercise, food intake or body weight can also affect menstruation.
If you have had sexual intercourse where pregnancy is possible, pregnancy may be one explanation for a missed period.
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Which describes what you have noticed?
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This can be normal, especially during adolescence. Noting the first day of your period for a few months can help you see your actual pattern.
Count the days from the first day of bleeding to the first day of the next period. A calendar or simple note can show variations more clearly than the date of the month. Hormone patterns and ovulation can still be irregular during adolescence.
Tell a healthcare professional about major changes, long gaps, pain, heavy bleeding or other symptoms. An app can estimate your next period, but cannot give a definite date.
If pregnancy is possible, take a pregnancy test three weeks after the last sexual intercourse with a risk of pregnancy. If your period is still absent and you do not know why, contact your GP or school nurse.
If pregnancy is possible, the date of the last sexual intercourse that could lead to pregnancy matters more than the date predicted by an app. A test three weeks after that event gives more useful information than symptoms or light bleeding.
A missed period can also have other explanations, including contraception, illness or changes in energy intake. Seek assessment if your period has been absent for around three months without a known explanation.
If your cycle has changed noticeably from what is usual for you, tell a school nurse or doctor.
Explain the change using an example, such as having much longer gaps between periods or bleeding between periods. Mention any medication, illness or changes in exercise or food intake.
This information helps the doctor choose appropriate investigations. You do not have to work out a hormonal explanation yourself or keep records for several months before asking for help.
What can you do now?
You can keep track of the first day of bleeding and any other symptoms. You do not have to find the cause yourself before asking for help.
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