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Published by Floriva · Updated 2026-04-29 · How Floriva checks its guides

Why Is My Period Late? A Diagnostic Guide

Why your period is late depends on the cause. Stress delays ovulation by days to weeks. PCOS and hypothalamic amenorrhea can stop it for months. Here is how to work through it.

A late period is almost always caused by delayed or absent ovulation, not by a problem with the period itself. The most common causes are stress, undereating, illness, thyroid dysfunction, PCOS, and the aftermath of hormonal contraception. Working through them in order (pregnancy first, then reversible causes, then conditions that require investigation) is the most efficient diagnostic path.

Most people searching "why is my period late" are hoping for one simple answer. The honest answer is that it depends on which part of the hormonal cascade is disrupted. Working through the possibilities in order is faster than guessing.

The mechanism is nearly always the same. The period is late because ovulation was late or did not happen. The period is triggered by progesterone withdrawal after the corpus luteum degrades, but the corpus luteum only exists if ovulation occurred. Late or absent ovulation is the proximate cause of almost every late period.

Step 1: Rule Out Pregnancy

If there's any possibility of pregnancy, take a test before working through other explanations. Modern urine pregnancy tests are highly sensitive (detecting hCG at 20-25 mIU/mL) and accurate from the first day of a missed period onward. A negative test 14+ days after unprotected sex is reliable.

If the test is positive, that's your answer. If the test is negative, move to Step 2.

Step 2: What Changed in the Past 4-6 Weeks

Most late periods have an identifiable trigger in the 2-4 weeks before the period was expected (the follicular phase, when the disruption would have had to occur to affect ovulation timing).

Ask yourself:

• Have you been significantly more stressed than usual (work pressure, a relationship crisis, bereavement, financial stress)?

• Have you changed your eating significantly, started a restrictive diet, reduced calories, or skipped meals repeatedly?

• Have you added or significantly increased exercise, particularly cardio volume?

• Have you been ill, especially with a fever?

• Have you traveled across time zones or had severe sleep disruption for more than a few days?

• Have you started or stopped any medication, particularly SSRIs, antipsychotics, antihypertensives, or hormonal contraception?

If yes to any of these: you've likely identified the cause. A single delayed cycle from a temporary stressor is normal and usually self-resolves the following cycle. No action needed beyond noting the pattern and taking a pregnancy test to confirm.

If no to all of these, or if this is a recurring pattern: keep working through the possibilities below.

Step 3: Have You Recently Stopped Hormonal Birth Control

Stopping combined oral contraceptives, the hormonal IUD, the implant, or hormonal injections can delay the return of ovulation for varying amounts of time.

Expected timelines after stopping:

• Combined OCP: Most people ovulate within 4-6 weeks; regular cycles typically return within 1-3 months. Post-pill amenorrhea lasting 6+ months is uncommon but recognized, particularly in people who had irregular cycles before starting.

• Hormonal IUD: Ovulation typically returns within 1-2 months (the levonorgestrel IUD acts primarily locally; the HPG axis is mostly suppressed).

• Implant: Ovulation can return within weeks, but can take 3-6 months.

• Injectable (Depo-Provera): Return to fertility is often delayed 6-12 months or longer. Depo is the outlier, regular cycles within 3 months of stopping are unusual.

If you stopped hormonal contraception within the past 3-6 months, a delayed period is likely post-hormonal cycle recovery. Give it time unless it's been longer than expected for the method, or if you have other symptoms that suggest a different cause.

Step 4: Thyroid Dysfunction

Both hypothyroidism and hyperthyroidism disrupt menstrual cycles. Hypothyroidism is more common and directly relevant. Elevated TSH stimulates prolactin secretion (they share regulatory overlap via TRH). Elevated prolactin suppresses GnRH. The cycle is disrupted through this indirect pathway as well as through direct thyroid hormone effects on ovarian function.

Signs suggesting thyroid involvement:

• Hypothyroidism: fatigue, weight gain despite normal eating, cold intolerance, dry skin or hair, constipation, slow heart rate

• Hyperthyroidism: weight loss with normal or increased appetite, heat intolerance, fast heart rate, tremors, anxiety

If these symptoms are present alongside late periods, a TSH test is the starting point. The standard normal range goes up to 4.5-5 mIU/L, but fertility-optimizing guidelines suggest targeting 1-2.5 mIU/L when trying to conceive.

Action: Ask your GP or gynecologist for TSH (and T4 if TSH is abnormal). Treating thyroid dysfunction typically normalizes cycles within 2-3 months.

Step 5: PCOS

Polycystic ovary syndrome (PCOS) is a common hormonal condition affecting reproductive-age people. Irregular or absent periods are a frequent presenting complaint. PCOS disrupts ovulation through androgen excess and often insulin resistance, which impair the normal follicle selection and LH surge mechanism.

Signs suggesting PCOS:

• Irregular cycles that have been irregular since adolescence (not a new change)

• Acne (particularly jawline/chin) that persists beyond teenage years

• Excess hair growth (hirsutism) on face, chin, abdomen, or inner thighs

• Thinning scalp hair (androgenic alopecia)

• Difficulty losing weight or weight concentrated in the midsection

• Multiple small follicles visible on pelvic ultrasound

PCOS does not resolve on its own. It requires management, but it is highly manageable through diet, exercise, and (if needed) medication. The diagnosis criteria require at least two of three: irregular ovulation, clinical or biochemical androgen excess, and polycystic ovarian morphology on ultrasound.

Action: If you have consistent menstrual irregularity plus at least one of the above signs, a gynecologist or endocrinologist visit is warranted. PCOS evaluation includes FSH, LH, testosterone, DHEA-S, and pelvic ultrasound.

Step 6: Hyperprolactinemia

Elevated prolactin from any cause suppresses the HPG axis. The most common cause is a prolactinoma, a benign pituitary adenoma that overproduces prolactin. It can also be caused by hypothyroidism (high TSH drives prolactin), certain medications (especially dopamine antagonists like some antipsychotics, metoclopramide, and older antihistamines), or have no identified cause (idiopathic).

Signs suggesting hyperprolactinemia:

• Milky nipple discharge (galactorrhea) when not breastfeeding

• Irregular or absent periods without another obvious explanation

• Headaches or visual changes (rare, associated with larger prolactinomas)

Action: Serum prolactin blood test. Values above 25 ng/mL warrant investigation. If a prolactinoma is found, treatment with a dopamine agonist (cabergoline is standard) typically normalizes prolactin and restores cycles within 1-2 months.

Step 7: Perimenopause (If Over 40, or Earlier With Relevant Family History)

Perimenopause is the hormonal transition preceding menopause, typically beginning in the mid-to-late 40s, though it can begin in the late 30s in some people. Rising FSH (as ovarian reserve declines) and increasingly erratic estrogen produce irregular cycle lengths, including both shorter and longer cycles.

Signs suggesting perimenopause:

• Over 40 (or close to it)

• New menstrual irregularity that wasn't present before

• Hot flashes or night sweats

• Sleep disruption

• Mood changes

If you are under 40 and experiencing irregular periods with hot flashes, this may be early perimenopause or premature ovarian insufficiency. Both warrant investigation.

Action: FSH on Day 2-3 of the cycle (if periods are still occurring). Elevated FSH confirms reduced ovarian reserve. This doesn't mean fertility is impossible, but it changes the clinical picture.

When One Cause Isn't Enough

Some late periods have more than one contributing factor. Stress combined with undereating is a common combination that amplifies HPG suppression. Thyroid dysfunction and PCOS can co-occur. Post-pill amenorrhea can also unmask underlying PCOS that was hidden while on the pill.

If the single-cause framework doesn't fit your pattern, or if you've been through this list and nothing explains recurring irregular cycles, a full hormonal panel is the most efficient next step. A useful baseline panel includes: TSH, prolactin, FSH, LH (both on Day 2-3 if cycles are occurring), estradiol, testosterone, DHEA-S, fasting insulin and glucose.

The Tracking Shortcut

If you have tracked ovulation (LH strips or BBT), the late period is much easier to interpret:

• BBT shows a clear biphasic pattern with late temperature rise: Ovulation was late, not absent. Period will follow 10-16 days after the rise. Cause is likely stress, illness, or lifestyle disruption.

• BBT shows no temperature rise at all: No ovulation occurred. The "period" when it arrives may actually be an anovulatory withdrawal bleed, lighter and more irregular than a true period.

• Multiple LH surge attempts before a confirmed ovulation: Possible PCOS pattern.

Without tracking data, you're reasoning from symptoms alone. With even a few cycles of BBT data, the pattern becomes significantly more legible.

What This Means for Floriva Users

Cycle length tracking across multiple months reveals whether late periods are isolated events or a recurring pattern. Ovulation tracking (LH strips or BBT) is the tool that distinguishes delayed ovulation (benign) from anovulation (warrants attention). If your cycles are consistently long or unpredictable, three months of tracking data is the most efficient thing you can bring to a gynecologist appointment.