The Menstrual Cycle: What Counts as Normal and When to See a Doctor

Your cycle has shifted, or a period lasted longer or was heavier than usual. The international definition of a normal cycle is a wide range, not exactly 28 days; what matters more is a change from your usual pattern and how much the bleeding and pain get in the way of your life. A few situations are emergencies, and some of them can look like an ordinary period.

When to see a doctor

Right away — 999 in the UK, 911 in the US, 112 in the EU:

• heavy bleeding with dizziness, light-headedness, feeling faint or fainting — whatever the number of pads or tampons;
• you might be pregnant (a late period is enough, even with a negative test) and have sudden, sharp, intense tummy pain, feel very dizzy or faint, sick or very pale: possible ruptured ectopic pregnancy (NHS);
• severe or worsening pelvic pain, sudden intense pain low in the tummy, with dizziness, fainting or a very high temperature (NHS, pelvic inflammatory disease);
• toxic shock syndrome signs while using a tampon or menstrual cup: confusion or slurred speech, blue, grey, pale or blotchy skin or lips, a rash that does not fade under a glass, difficulty breathing, large areas of peeling skin. Remove the tampon or cup and tell the call handler you were using one (NHS).

The same day — 111 or your GP in the UK, urgent care in the US:

• high temperature with chills and muscle aches, vomiting and diarrhoea, or a rash that feels like sandpaper while using a tampon or cup: remove it and say you were using one (NHS);
• you might be pregnant, even without a positive test, and have pain low down on one side, bleeding or a brown watery discharge, or pain in the tip of the shoulder. Bleeding from an ectopic pregnancy is easy to take for a period (NHS);
• pelvic pain that comes and goes or pain deep inside during sex — possible pelvic inflammatory disease; urgent if you might be pregnant (NHS);
• period or pelvic pain that is severe or worse than usual and not eased by painkillers (NHS).

Book an appointment, and do not put it off:

• any bleeding after the menopause (a year or more without periods) — even once, even spotting or a pink or brown discharge. In the UK you should not wait more than two weeks to see a specialist (NHS);
• bleeding between periods or after sex;
• heavy periods that affect your life;
• three missed periods in a row with a negative pregnancy test, no periods by 15, or periods that have become irregular (NHS);
• pain that stops you doing normal things, pain during sex, peeing or pooing.

What is happening

The normal range (FIGO, 2018). The International Federation of Gynecology and Obstetrics sets the limits from population studies of women aged 18–45:

Bleeding between regular periods falls outside the norm. Many popular sources, including the NHS, give 21–35 days; we use FIGO because that is the definition gynaecologists use internationally. Teenagers and people in their forties vary more: periods start at about 12 on average, and a regular cycle usually settles by 16–18 (NHS, UK).

What "heavy" means. The NHS lists: changing a pad or tampon every 1–2 hours or emptying a cup more often than recommended, clots bigger than about 2.5 cm, bleeding through to clothes or bedding, periods longer than 7 days, constant tiredness or breathlessness. The 80 ml threshold is a research definition; in the clinic, the effect on your life matters more (FIGO). Causes include fibroids, endometriosis, adenomyosis, bleeding disorders such as von Willebrand disease, anticoagulant medicines, and rarely womb cancer (NHS). Heavy periods are a common cause of iron deficiency.

Missing periods. The first question is pregnancy. Other causes: stress, perimenopause (usually 45–55, UK), polyendocrine metabolic ovarian syndrome — PMOS (formerly PCOS) — sudden weight loss, being overweight, a lot of exercise, hormonal contraception, breastfeeding (NHS). Thyroid problems also disturb the cycle: check your thyroid if you are always tired.

Pain. Some cramping is common. Pain that disrupts ordinary life is not something to simply put up with: causes include endometriosis, adenomyosis, fibroids and pelvic inflammatory disease, and an IUD in its first 3–6 months (NHS).

Tampons and cups. Toxic shock syndrome is rare but life-threatening. The FDA (US) advises changing a tampon every 4–8 hours, not wearing one longer than 8 hours, and using the lowest absorbency you need; the NHS says to follow the product instructions.

What to do

• Keep a diary for three cycles or more: first day, length, how often you change products, clots, pain, bleeding in between. It turns a vague "something is off" into a concrete conversation: bring questions to the doctor.
• Late period: take a pregnancy test first.
• Heavy periods: ask about a blood test for iron.
• Pain: heat, gentle activity and paracetamol or ibuprofen help many people; who should avoid ibuprofen is on paracetamol vs ibuprofen.
• Tampons and cups: change on time and choose the lowest absorbency that works.

When this is not the case

• Hormonal contraception changes bleeding: the bleed in a pill-free week is not a natural cycle, and some methods stop periods altogether — see contraception effectiveness by method.
• After emergency contraception the next period may come earlier or later; if it is more than a week late, lighter than usual or unusually painful, take a pregnancy test — emergency contraception: how it works.
• After birth and while breastfeeding periods may be absent for months.
• In your forties cycles change with perimenopause; similar changes before 40 need a doctor's assessment.
• Trying to conceive: fertility and age.

Sources

• Munro et al., FIGO 2018 — Systems for abnormal uterine bleeding
• Jain et al., FIGO Systems 1 and 2 (2023)
• NHS — Periods
• NHS — Heavy periods
• NHS — Stopped or missed periods
• NHS — Period pain
• NHS — Toxic shock syndrome
• FDA — Tampons
• NHS — Ectopic pregnancy
• NHS — Pelvic inflammatory disease
• NHS — Postmenopausal bleeding

Checked: 24 September 2026