When is your next period likely to arrive?

A 28-day cycle is within the normal range, but it is not the default. A study of 612,613 menstrual cycles by Bull et al. (2019) found that only 13% of cycles are exactly 28 days. The mean cycle length is 29.3 days, and the normal range defined by ACOG is 21-35 days for adult women. The "28-day" figure became the cultural standard partly because early oral contraceptive pill cycles were designed around 28-day packs. Your cycle does not need to be 28 days to be normal.

Cycle-to-cycle variation of up to 7 days is generally considered within the normal range. ACOG classifies cycles as regular when variation is 1-4 days between consecutive cycles, and mildly irregular when variation is 5-7 days. Approximately 50-60% of reproductive-age women have cycle variation of 4 days or less. Variation of 8 or more days is considered moderately irregular and may warrant discussion with a healthcare provider, particularly if it represents a change from a previously regular pattern.

Yes. Cycle length follows a predictable age-related pattern. Bull et al. (2019) found that younger women (18-24) have slightly longer average cycles (~30 days) which gradually shorten through the 30s and 40s. By age 40-44, mean cycle length drops to approximately 27.5 days. This shortening is driven by changes in the follicular phase as the ovarian reserve declines. In the years approaching menopause (perimenopause, typically beginning in the mid-40s), cycles become progressively more variable before eventually ceasing.

Prediction accuracy decreases as cycle variability increases, but estimates are still possible with wider confidence windows. If your cycle varies by 2-3 days, a prediction can narrow the expected date to a window of about 5 days. If your cycle varies by 7+ days, the prediction window expands to 10-14 days. The more cycle data you provide (6 recent cycles rather than 1-3), the more accurate the average. For highly irregular cycles (variation of 15+ days), prediction becomes unreliable, and consulting a healthcare provider about the underlying cause is recommended.

Yes, fundamentally. Hormonal contraception creates an artificial cycle or suppresses menstruation. If you are using hormonal contraception, this calculator's prediction reflects the contraceptive schedule rather than your natural cycle. To understand your natural cycle, you need to have been off hormonal contraception for at least 3-6 months, as it can take several cycles for the body to re-establish its natural pattern.

Yes. The hypothalamus, which controls the hormonal signals driving ovulation, is sensitive to cortisol. Under chronic or acute stress, elevated cortisol can suppress GnRH, delaying or preventing ovulation. The effect ranges from a few days' delay to complete cessation of periods (functional hypothalamic amenorrhea), which affects approximately 3-5% of reproductive-age women. The effect is typically reversible once the stressor resolves, with cycles returning to baseline within 1-3 months.

ACOG recommends consulting a healthcare provider if: your cycles are consistently shorter than 21 days or longer than 35 days, you miss periods for 3 or more consecutive months, your periods last longer than 7 days, you experience bleeding between periods, or your periods are so heavy that you soak through a pad or tampon every hour for several consecutive hours. A single unusual cycle is rarely concerning, but a persistent pattern warrants investigation to rule out conditions such as polycystic ovary syndrome (PCOS), thyroid disorders, or endometriosis.

The most common causes of a late period (after ruling out pregnancy) include: stress, significant weight change, excessive exercise, illness or travel, changes to hormonal contraception, perimenopause (if aged 40+), polycystic ovary syndrome (PCOS, which affects approximately 8-13% of reproductive-age women according to WHO), thyroid disorders, and certain medications. A single late period is usually not cause for concern if there is an identifiable trigger. Consistently late or absent periods (amenorrhea) should be evaluated by a healthcare provider.

The Apple Women's Health Study, a collaboration between Harvard T.H. Chan School of Public Health, NIH, and Apple, is the largest prospective menstrual cycle study ever conducted, with over 50,000 participants. Key published findings include: confirmation that the 28-day "standard" cycle is a minority pattern, documentation of significant cycle variability even among women who consider their cycles regular, and evidence that many women with clinically irregular cycles are unaware their pattern falls outside the normal range. The study is ongoing and continues to publish findings.