Why is my period late? Common reasons, and when to see a doctor

In short: a period can be late for lots of reasons besides pregnancy: stress, losing or gaining weight, exercising hard, some kinds of contraception, breastfeeding, and conditions like PCOS or thyroid problems. Cycles also simply vary a little from month to month. Late or missed periods are usually nothing to worry about, but if you've missed three in a row, or a late period comes with other changes, see a doctor (NHS).

First, is it actually late?

"Late" depends on your own pattern, not on 28 days. A cycle anywhere from 21 to 35 days long counts as normal (NHS), and only 13% of cycles in a study of more than 612,000 were exactly 28 days long (Bull et al., 2019). The same study found that one person's cycles typically vary by a couple of days or so from month to month.

So if your cycles usually run 32 days, day 30 isn't late yet. If you're not sure what your usual length is, our free period calculator estimates your next date from your last period and average cycle. More on what counts as normal: how long is a normal menstrual cycle?

Could I be pregnant?

If you've had sex, this is the first thing to rule out. Most home pregnancy tests work from the first day of a missed period. If you don't know when your period was due, test at least 21 days after you last had unprotected sex (NHS).

A positive result is almost always right. A negative one is less certain, especially if you tested early. If it's negative and your period still hasn't come, wait a few days and test again, and if that's negative too, talk to a doctor (NHS).

Common reasons your period is late

The NHS lists these as the usual causes of late or missed periods (NHS, NHS):

• Stress. A hard month at work, exams, grief, a move: stress and anxiety can delay or stop a period.
• Losing weight suddenly, or eating much less than your body needs.
• Being overweight, or gaining a lot of weight.
• Doing too much exercise. Hard training can delay or pause periods.
• Hormonal contraception. The progestogen-only pill, the contraceptive injection and the hormonal IUS can make periods irregular, lighter or stop them altogether. The combined pill can too.
• Breastfeeding, which can keep periods away for a while after having a baby.
• PCOS. Polycystic ovary syndrome, which the NHS now calls PMOS (polyendocrine metabolic ovarian syndrome), is a common cause of irregular or missed periods.
• Thyroid problems, either an overactive or an underactive thyroid.
• Other medical conditions, such as diabetes or heart disease.
• Perimenopause, the years before menopause, usually between 45 and 55, when periods get less regular before they stop.
• Puberty. When periods first start, it's normal for them to be irregular. It can take up to three years for them to settle (Cleveland Clinic).

Often there isn't one dramatic cause. Sometimes a cycle just runs long, especially when ovulation happens later than usual. The days before ovulation are the part of the cycle that varies most, so a later ovulation means a later period (Bull et al., 2019).

What you can do now

• Take a pregnancy test if there's any chance you could be pregnant.
• Look back at the last few weeks. Illness, travel, poor sleep, a big change in eating or training, or a stressful stretch can all explain one late cycle.
• Check your contraception. If you've started, stopped or switched a method recently, that may be the reason.
• Note it down. Write the date your period finally starts, and anything else you've noticed. A few months of records makes any pattern much easier to see, for you and for a doctor.

One late period on its own is common. A pattern is what's worth paying attention to.

When to see a doctor

See a GP or doctor if (NHS, NHS):

• you've missed your period three times in a row
• your periods haven't started by 15
• your periods have become irregular
• a missed or late period comes with weight gain or loss, tiredness, extra facial or body hair, or dry or oily skin
• you have bleeding between periods
• your periods are irregular and you're finding it hard to get pregnant

A doctor will usually ask about your health and cycles, and may suggest blood tests to check your hormones or thyroid. They might refer you to a specialist for scans. Treatment depends on the cause, and sometimes none is needed (NHS).

Get urgent help if you've missed a period and have pain low down on one side of your tummy, bleeding, or pain in the tip of your shoulder: these can be signs of an ectopic pregnancy. Call your local emergency number if the pain is sudden and severe, or you feel very dizzy or faint (NHS).

Keep track, gently

A late period is much less stressful when you know your own rhythm. Periodiva learns your cycle as you log it, so predictions follow your body rather than a textbook 28 days. Until the app launches, our free period calculator runs on your device and saves nothing.

Read next: how long is a normal menstrual cycle? Or see the stretch just before your period: the luteal phase.