Why Is My Period Late or Irregular?

Day 31. You open the app, then the calendar, then the app again, as if one of them had changed its mind. You carry a spare pair of underwear in your bag and a low-level worry you can’t name.

What counts as irregular

The numbers vary a little by source. The NHS³ and Cleveland Clinic⁴ say a cycle is irregular when the time between periods is fewer than 21 days or more than 35 days. OWH² uses a wider window, less than 24 or more than 38 days. Cleveland Clinic also counts missing three periods in a row, a cycle that varies by more than nine days, unusually heavy or light flow, and periods that last longer than seven days.

Cleveland Clinic⁴ says a typical cycle runs 28 days, and that normal cycles range from 21 to 35.

The common reasons

The NHS¹ lists these for late or missed periods, and the lists from OWH² and Cleveland Clinic⁴ overlap:

• Pregnancy. The first one on the NHS¹ list.
• Stress and anxiety. The NHS¹ says a GP can help with advice or therapy to reduce stress.
• Weight changes. The NHS¹ names sudden weight loss and being overweight. OWH² adds eating disorders, especially anorexia nervosa, and obesity.
• Exercise. Doing too much, the NHS¹ says.
• Breastfeeding.
• Perimenopause. The NHS¹ says it usually starts between 45 and 55. Our article on perimenopause covers the signs.
• Hormonal contraception. The NHS³ lists the progestogen-only pill, the injection and the hormonal coil.
• Medicines. Cleveland Clinic⁴ lists birth control pills, steroids and blood thinners.
• Medical conditions. The NHS³ lists polyendocrine metabolic ovarian syndrome (PMOS, which it says used to be called polycystic ovary syndrome, PCOS) and thyroid problems. Cleveland Clinic⁴ adds endometriosis and ovarian insufficiency. OWH² adds uncontrolled diabetes and primary ovarian insufficiency.

Read that list as a map, not a diagnosis. The pattern matters more than any single late month.

The play

Try this: a three-month period log

Write the first day of each period, how many days it lasted, and whether the flow was light, usual or heavy.

Add a line each month for what was different: a stressful stretch, travel, a new medicine, a change in exercise or weight, a bad sleep run.

The NHS³ suggests an app or a diary, and OWH² says a diary or calendar can help your doctor or nurse spot a problem. Three months of notes turns “my periods are weird” into dates.

What a clinician does

Cleveland Clinic⁴ says doctors start with a physical exam and may order a pelvic ultrasound to look for fibroids, polyps or cysts. Treatment depends on the cause. It lists hormonal birth control to regulate cycles, and the NHS³ gives the example of the combined pill for PMOS and hormone medicine for thyroid problems. If you’re trying to get pregnant, the NHS says irregular cycles make it harder to know when you ovulate, and it suggests sex every 2 or 3 days throughout your cycle.

If heavy flow is part of the picture, see our article on whether your period is too heavy. For cramping, period cramps has more.

When to get checked

The NHS¹ says to see a GP if you’ve missed your period 3 times in a row or your periods have become irregular. The NHS also lists periods that last longer than 7 days, and bleeding between periods. OWH² says to contact a doctor if you get irregular periods after having normal cycles, or if you haven’t had a period for three consecutive months and you’re not pregnant or breastfeeding. Cleveland Clinic⁴ adds severe cramping, unusually heavy bleeding or large clots, and foul-smelling discharge.

Book an appointment if any of these fit:

• You’ve missed three periods in a row
• Your cycles became irregular after being regular
• Your periods last longer than 7 days
• You’re bleeding between periods
• Your periods are accompanied by weight gain, tiredness, facial hair or skin changes, which the NHS³ lists as reasons to see a GP
• You’re trying to get pregnant and your cycles are unpredictable

Day 31 becomes day 32 and then your period arrives, or it doesn’t. Either way, you’ll have the dates.