Irregular and Absent Periods: What Causes Them, Which Tests Matter and What Treatment Can Fix
A menstrual cycle is the visible end of a conversation between three organs. The hypothalamus releases GnRH in pulses, the pituitary responds with FSH and LH, and the ovary grows a follicle, releases an egg and then produces progesterone. If ovulation happens, progesterone rises and falls, and a period follows about two weeks later. If ovulation does not happen, progesterone never rises, and the bleeding becomes unpredictable or stops.
Almost every irregular cycle comes down to that: ovulation is not happening reliably. The useful question is not how to make bleeding regular, but why ovulation stopped, because the answer determines the treatment and sometimes matters well beyond the cycle.
⚠️ An important correction, because this is the most widespread misunderstanding in the subject. The combined pill does not restore a natural cycle or correct a hormone imbalance. It does the opposite: it suppresses the hypothalamic-pituitary-ovarian conversation, stops ovulation, and replaces the natural cycle with a predictable withdrawal bleed during the pill-free week. That bleed looks reassuring and tells you nothing about your own hormones. When the pill is stopped, the original irregularity returns unchanged, because nothing underneath was treated. The pill is a legitimate and useful treatment for the symptoms of an irregular cycle. It is not a cure, and it should not be a substitute for finding the cause.
📅 What counts as normal, and what does not
| Parameter | Normal range | Outside it |
|---|---|---|
| Cycle length | 21 to 35 days in adults | Under 21 days is polymenorrhoea; over 35 days is oligomenorrhoea |
| Variation between cycles | Up to 7 to 9 days | More than that counts as irregular even if each cycle is within range |
| Days of bleeding | 2 to 7 days | Under 2 days is hypomenorrhoea; over 8 days needs assessment |
| Blood loss | Up to about 80 mL | Flooding, clots, changing protection hourly, or anaemia |
| Absence of periods | — | 3 months if previously regular, or 6 months if previously irregular, is amenorrhoea |
Two situations where irregularity is expected and normal: the first two years after periods start, and the years approaching menopause. Outside those, an irregular cycle has a cause.
🧪 The workup, in the order it is done
🤰 First, always, a pregnancy test. Pregnancy is the commonest cause of a missed period, including in women who believe it is impossible. No investigation proceeds before it is excluded, and no hormone treatment is started.
After that, a small set of blood tests identifies the cause in the large majority of cases:
| Test | What it identifies |
|---|---|
| TSH | Thyroid disease. Both under- and overactivity disturb cycles, and it is the easiest cause to fix |
| Prolactin | Hyperprolactinaemia, often from a small pituitary tumour or from medication |
| FSH and LH | High FSH points to the ovary failing; low FSH and LH point to the hypothalamus switching off |
| Total testosterone, SHBG, free androgen index | Androgen excess, as in polycystic ovary syndrome |
| 17-hydroxyprogesterone | Late-onset congenital adrenal hyperplasia, which mimics PCOS |
| Pelvic ultrasound | Ovarian appearance, endometrial thickness, fibroids, polyps |
Blood tests for FSH, LH and progesterone are timed to the cycle where there is one, so it is worth asking which day to attend rather than going at random.
🧩 The causes, and what each one looks like
🟣 Polycystic ovary syndrome
The most common cause of irregular cycles in women of reproductive age, affecting roughly one in ten. Diagnosed when two of three features are present: irregular or absent ovulation, clinical or biochemical androgen excess, and polycystic ovarian appearance on ultrasound.
The picture is cycles that come every 40 to 90 days or stop altogether, acne and excess hair growth on the face, chest or abdomen, thinning hair at the crown, difficulty losing weight, and difficulty conceiving. Insulin resistance underlies much of it, which is why weight loss of 5 to 10 percent frequently restores ovulation on its own. Our guide to polycystic ovary syndrome covers it in more detail.
🔵 Thyroid disease
An underactive thyroid causes heavy or infrequent periods along with fatigue, weight gain, cold intolerance and dry skin. An overactive one causes light or absent periods. Both are diagnosed on one blood test and both resolve on treatment, which is why TSH is checked before anything more elaborate.
🟡 High prolactin
Prolactin suppresses GnRH, so raised levels stop ovulation. The classic combination is absent periods with milky discharge from the nipples, sometimes with headache or visual field loss if a pituitary tumour is large. Causes also include medications: antipsychotics, metoclopramide, domperidone and some antidepressants. Treatment with a dopamine agonist such as Dostinex (Cabergoline) or Parlodel (Bromocriptine) restores cycles in most cases.
🔴 Hypothalamic amenorrhoea
🦴 This is the cause that must not be masked with a pill. When energy intake is too low relative to output, from restrictive eating, heavy training, rapid weight loss or severe stress, the hypothalamus switches off GnRH pulses to conserve resources. Periods stop, and oestrogen falls to postmenopausal levels.
The consequence is bone loss during the years when peak bone mass should be building, and it can be irreversible. Stress fractures in a young athlete with absent periods are the classic presentation. Prescribing a combined pill produces a monthly bleed that looks like recovery while the underlying energy deficit continues, and the pill does not reliably protect the bone in this situation. The treatment is restoring energy availability: eating more, training less, and gaining weight. That is harder than a prescription and it is the only thing that works.
⚫ Premature ovarian insufficiency
The ovaries stopping before the age of 40, affecting about one woman in a hundred. Suggested by absent or erratic periods with hot flushes, night sweats and vaginal dryness, and confirmed by a raised FSH on two occasions. It needs hormone replacement, continued until the average age of menopause, for bone and cardiovascular protection rather than only for symptoms.
⚪ Structural and other causes
- 🩸 Fibroids and polyps — heavy or prolonged bleeding rather than irregularity, and bleeding between periods
- 🦷 Intrauterine adhesions — absent periods following a uterine procedure or infection
- 💊 Medication — hormonal contraception itself, antipsychotics, chemotherapy, long-term steroids, opioids
- 🧬 Adrenal and other endocrine disease, and coeliac disease, which is worth considering where there are gut symptoms
💊 Treatment: matched to the cause and to the goal
The right treatment depends on two questions: what is causing it, and whether pregnancy is wanted now. Those two answers point in different directions, which is why a single approach for everyone does not exist.
🔹 If the cause is correctable, correct it
Thyroid replacement, a dopamine agonist for high prolactin, weight loss in PCOS, restoring energy intake in hypothalamic amenorrhoea. These treat the disorder rather than the bleeding, and cycles usually return on their own.
🔹 A progestogen course to induce a bleed
Where ovulation is not happening, oestrogen continues to thicken the endometrium with no progesterone to organise and shed it. Over years that thickening carries a risk of endometrial hyperplasia and cancer, so regular shedding matters medically and not only for convenience.
Duphaston (Dydrogesterone 10 mg) is used for this, typically 10 mg once or twice daily for 10 to 14 days of each cycle or each month, with bleeding following two to seven days after the last tablet. Its advantage over a combined pill here is that it does not suppress the ovulatory axis, so it protects the endometrium without switching the system off, and it does not prevent pregnancy. Other progestogens used the same way include Provera (Medroxyprogesterone) and Aygestin (Norethindrone).
🔹 Combined hormonal contraception
A reasonable choice where predictable bleeding, contraception, acne control and reduced excess hair growth are all wanted at once, which describes many women with PCOS. Options in our birth control category include Yasmin (Drospirenone and Ethinyl Estradiol), whose progestogen has an anti-androgenic effect useful for acne and hirsutism, and Mircette (Desogestrel and Ethinyl Estradiol).
⛔ When a combined pill cannot be used. Migraine with aura, at any age, because of stroke risk. Smoking at age 35 or over. Previous venous thrombosis or a known clotting disorder. Uncontrolled high blood pressure. Current or past breast cancer. Active liver disease. Certain migraines and clot histories in close relatives warrant discussion. Where a combined pill is unsuitable, progestogen-only options achieve cycle control without the oestrogen risk.
🔹 If pregnancy is wanted
Here contraception is obviously the wrong tool, and the goal is to induce ovulation rather than to produce a bleed. Clofert (Clomiphene Citrate 50 mg) and Fertomid (Clomiphene Citrate) block oestrogen feedback at the hypothalamus, which raises FSH and stimulates a follicle. Typically 50 mg daily for five days starting early in the cycle, increased if needed, with monitoring to confirm ovulation and to watch for multiple follicles.
Femara (Letrozole 2.5 mg) is now often preferred in PCOS, where trials found higher ovulation and live birth rates than clomiphene, with fewer twin pregnancies. Either way, ovulation induction belongs under medical supervision, not self-medication, because of the risk of ovarian hyperstimulation and multiple pregnancy.
🥗 What lifestyle actually changes
- Weight, in either direction
- In PCOS, losing 5 to 10 percent restores ovulation in a substantial proportion of women. In hypothalamic amenorrhoea, gaining weight does the same. Both work through the same mechanism of energy availability.
- Enough food for the training load
- The problem in athletes is not exercise itself but the gap between intake and expenditure. Cycles return when the gap closes.
- Sleep and shift work
- Circadian disruption affects the hormone pulses that drive the cycle; night shift work is associated with more irregularity.
- Stopping smoking
- Smoking brings menopause forward by one to two years and worsens cycle irregularity.
- Tracking
- Three months of dates and flow, brought to the appointment, is more useful than any recollection and often makes the pattern obvious.
On supplements, the honest position is that inositol has modest evidence in PCOS, vitamin D correction helps if deficient, and the rest of what is marketed for hormone balance has none.
🚨 Bleeding that needs assessment rather than regulation
- Bleeding after sex, or between periods
- Any bleeding after the menopause — always investigated
- Soaking through protection hourly, or passing large clots
- Periods with breathlessness, palpitations or extreme fatigue — likely anaemia
- Severe pain not controlled by ordinary painkillers, or pain that has changed in character
- Absent periods with milky nipple discharge, headache or changes in vision
- Absent periods with hot flushes or night sweats under 40
- Absent periods in an athlete or after weight loss — bone density needs assessing, not a pill prescribing
- No periods by age 15, or no breast development by 13
❓ Frequently asked questions
Does the pill fix an irregular cycle?
It controls the symptom, not the cause. The combined pill suppresses ovulation and replaces the natural cycle with a withdrawal bleed, so the bleeding becomes predictable while the underlying problem is untouched. Irregularity returns when the pill is stopped.
How irregular is too irregular?
Cycles of 21 to 35 days with up to about a week of variation are normal. Beyond 35 days, under 21 days, variation of more than nine days, or no period for three months if previously regular, all warrant assessment.
Which tests should I ask for?
A pregnancy test first, then TSH, prolactin, FSH and LH, and testosterone with SHBG. A pelvic ultrasound looks at the ovaries and the lining of the womb. That small set identifies the cause in most cases.
Why does it matter if I only bleed twice a year?
Because without ovulation there is no progesterone to shed the lining, so oestrogen keeps thickening it. Over years that carries a risk of endometrial hyperplasia and cancer, which is why regular shedding is induced with a progestogen course even when pregnancy is not wanted.
My periods stopped when I started training hard. Is that a problem?
Yes, and it should not be treated with a pill. Low energy availability switches off the hormone pulses that drive the cycle, oestrogen falls to postmenopausal levels, and bone is lost during the years peak bone mass should be building. The treatment is eating more and training less.
Can losing weight bring my periods back?
In polycystic ovary syndrome, yes: losing 5 to 10 percent of body weight restores ovulation in a substantial proportion of women, because insulin resistance drives much of the problem.
What does milky discharge from the nipples mean?
Usually raised prolactin, which suppresses ovulation and stops periods. It can come from a small pituitary tumour or from medicines such as antipsychotics and metoclopramide, and it is treated with a dopamine agonist. Headache or visual changes alongside it need prompt assessment.
How does a progestogen course work?
Taken for 10 to 14 days of the cycle, it organises the thickened lining so that it sheds when the tablets stop, with bleeding two to seven days later. Unlike a combined pill it does not suppress ovulation, and it does not prevent pregnancy.
Who cannot take the combined pill?
Anyone with migraine with aura, smokers aged 35 or over, and those with previous venous thrombosis, a clotting disorder, uncontrolled high blood pressure, breast cancer or active liver disease. Progestogen-only options can control cycles without the oestrogen risk.
I want to conceive. Will cycle regulation help?
Not contraception, which prevents exactly what is wanted. The goal becomes inducing ovulation, with clomiphene or letrozole under medical supervision and monitoring, because of the risk of multiple pregnancy and ovarian hyperstimulation.
📑 Sources and editorial
- Gynaecology and reproductive endocrinology guidance on the evaluation of oligomenorrhoea and amenorrhoea
- Rotterdam criteria and international evidence-based guidance on polycystic ovary syndrome
- Guidance on functional hypothalamic amenorrhoea, including bone health and the limits of hormonal contraception in this setting
- Trials comparing letrozole with clomiphene for ovulation induction in polycystic ovary syndrome
- Prescribing information for dydrogesterone, medroxyprogesterone, clomiphene citrate, letrozole and cabergoline
- Medical eligibility criteria for combined hormonal contraceptive use, including migraine with aura and smoking over 35
- Related reading: polycystic ovary syndrome, menopause and long-term health
- Related products: Duphaston (Dydrogesterone), Clofert (Clomiphene), Fertomid (Clomiphene), women's health category, birth control category
- RXshop Editorial Team — reviewed by Anita Sharma, MD, FACOG — Obstetrician-Gynecologist & Women's Health Specialist
Medical Disclaimer: The information in this article is for educational and informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek guidance from a qualified healthcare provider with any questions you may have regarding a medical condition, and before starting, stopping or changing any medication.