Periods are classified as occurring every 28 days on average but in reality, it can be shorter or longer than this - anything from 21 days to 40 days. For most women, they can predict which day they expect to have a period, and most use an app or diary to do this.
Periods are termed irregular when the expected date of bleeding becomes difficult to predict – earlier or later than expected. If this continues for three months or more, you should see your doctor to consider any necessary tests.
If it’s late once, then the first thing we would suggest is to take a pregnancy test.
Periods can be irregular for several reasons, so blood tests are done to account for the possibilities.
Approaching menopause can cause your periods to become irregular. If you are around the expected age and experiencing symptoms, blood tests are not usually required. However, if you are younger than 45 with irregular periods and other symptoms of menopause, blood tests for FSH (follicle-stimulating hormone) and oestrogen can help point to premature ovarian failure or premature ovarian insufficiency.
Prolactin is a hormone that, when raised, can lead to irregular periods. It is usually involved in pregnancy and milk production for breastfeeding, so high levels can affect fertility and your chances of getting pregnant.
Disruptions to your thyroid hormones (TSH or thyroid stimulating hormone, and free T4), can cause irregular periods, especially in hypothyroidism (underactive thyroid gland).
Your testosterone levels are useful in diagnosing a condition called PCOS (polycystic ovary syndrome), which affects the release of egg at ovulation and can therefore lead to irregular periods and infertility.
Your periods can be irregular for the first few years of your periods starting. Childbirth, miscarriage or pregnancy termination can also disrupt periods for a few months afterwards. Progestogen-only contraceptives such as the progestogen-only pill, implant or injection can cause periods to become irregular, lighter or even stop altogether. Contraceptives that also contain oestrogen, like the standard contraceptive pill, are often used to help regulate periods, but they can also disrupt timing. This can take several months to settle after stopping the contraceptive.
More recently we have become aware of side effects of the COVID-19 vaccine leading to irregular periods too, but that is usually short-lived and not thought to have any long-term consequences or effect on your fertility.
It’s important not to overlook how sensitive hormones are to stress – either over-working, big events like a wedding or bereavement, or significant illness. Excessive exercise and sudden weight loss or weight gain can also lead to changes in the length of your menstrual cycle.
If the gap between your periods seems shorter at times, it’s important to consider whether you are actually experiencing a period, or if it’s spotting. Polyps in the vaginal passage, tissue damage from infections like thrush or other conditions related to the reproductive organs can causing bleeding between periods or after sex. In this case, your doctor may suggest an internal examination and using a speculum to visualise the vaginal passage, and they may send you for an ultrasound scan.
It’s important to rule out a sexually transmitted infection (STI) such as gonorrhoea or chlamydia, so consider getting an STI check at your local clinic.
Sometimes we don’t find a cause for irregular, but given a few months, they become regular again – that's why the usual guide is to wait at least 3 months before seeing your doctor, unless you have specific concerns or other symptoms.
If you’re having irregular periods and it’s affecting your ability to become pregnant you should certainly speak to your doctor. If your periods occur more frequently than every 21 days, you’re having less than 4 periods a year or you haven’t had a period in 6 months, you should contact your doctor for further tests and investigations. If you have bleeding after sex or between periods, do book an appointment.
In addition, if you have noticed a sudden change in your period and you are younger than 45 years old, and there is concern regarding early menopause or premature ovarian insufficiency, then you should speak to your doctor.
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