Perimenopause Cycle Changes: STRAW+10 Staging in Plain English

The cycle changes that announce perimenopause are easy to dismiss as random — a short cycle here, a skipped one there, a week of spotting before what should have been a period. They are not random. They follow a predictable trajectory that researchers have mapped in detail through the SWAN (Study of Women’s Health Across the Nation) cohort and codified in the STRAW+10 staging system.

This post translates the staging into plain English: what to expect from your cycle from your mid-30s onward, what hormones are doing behind the scenes, and where the line is between normal and worth investigating.

What perimenopause actually is

“Perimenopause” means the transition from regular reproductive cycling to menopause. It is defined backward — once you have gone 12 consecutive months without a period, the day after your last period is your menopause date, and everything before that becomes perimenopause in retrospect.

The transition typically lasts 4–8 years. Average age at final menstrual period in the US is 51. Average age at first noticeable cycle change is around 47, though it can begin in the late 30s.

STRAW+10: the staging system

The Stages of Reproductive Aging Workshop +10 (Harlow SD, Gass M, Hall JE et al., “Executive summary of the Stages of Reproductive Aging Workshop +10,” Menopause 2012) is the international standard for describing where someone is in the reproductive aging continuum. It uses 10 stages anchored to the final menstrual period (FMP).

Simplified:

The two stages that matter most for cycle tracking are -2 (early transition) and -1 (late transition).

Stage -2: the early transition

Defined by persistent variation of seven or more days in cycle length, recurring within 10 cycles. Practical examples:

• Your cycles used to be 28–29 days. Now they range from 24 to 33.
• You skipped one cycle last year and one this year.
• Your luteal phase shortens; spotting starts a week before bleeding.

Hormone profile in stage -2:

• FSH: Trending up but variable. A single FSH value is not diagnostic.
• AMH (anti-Müllerian hormone): Declining steadily. AMH below 1.0 ng/mL is consistent with late reproductive or early transition.
• Estradiol: Still in the reproductive range overall, but with sharper peaks and irregular troughs. Some cycles see estradiol surges higher than your 30s did, which is why breast tenderness and heavy bleeding can intensify during this stage.
• Inhibin B: Falling. Lower inhibin removes feedback on FSH, which is why FSH starts rising.

Stage -1: the late transition

Defined by an interval of amenorrhea (no period) of 60 days or more, at least once. You may have periods every two months, then a four-month gap, then back to monthly.

Hormone profile in stage -1:

• FSH: Frequently above 25 IU/L, often above 30.
• Estradiol: Highly variable — can spike to premenopausal levels then drop suddenly.
• Vasomotor symptoms (hot flashes, night sweats) typically begin in this stage.

Average duration of stage -1 is 1–3 years before FMP.

What SWAN tells us about cycle patterns

The Study of Women’s Health Across the Nation (Santoro N, “Perimenopause: from research to practice,” J Womens Health 2015) followed over 3,000 women for 17+ years through the menopause transition. Key findings on cycle changes:

• Cycle length variability is the earliest reliable marker. Variability of seven or more days starts on average about 7 years before FMP.
• Cycle length both shortens and lengthens. In the early transition, cycles often shorten (luteal phase intact, follicular phase short due to elevated FSH recruiting follicles faster). In the late transition, cycles lengthen as ovulation becomes more sporadic.
• Heavy bleeding is common. Roughly one in three women in the menopause transition reports flooding or clots. This is driven by anovulatory cycles where unopposed estrogen builds the endometrium without progesterone to organize shedding.
• Skipped ovulation does not equal infertility. Spontaneous pregnancy rates drop sharply after 40 but are not zero until FMP.

How perimenopause feels day-to-day

The bleeding and cycle changes tend to attract clinical attention, but the symptom cluster also includes:

• Sleep disruption — often the first symptom, sometimes years before bleeding changes
• Hot flashes and night sweats — typically late transition and early postmenopause
• Mood lability, irritability, anxiety — driven by estrogen variability, not just decline
• Brain fog, word-finding difficulty — usually mild and reversible, peaking in late transition
• Joint aches — increasingly recognized; not “just aging”
• Vaginal dryness — late transition and postmenopause

Symptom intensity and duration vary hugely. About 25% of women have minimal symptoms; another 25% have severely disruptive symptoms; the middle 50% have moderate symptoms that come and go.

When irregular is normal vs. when to investigate

Normal in perimenopause:

• Cycle length variation of 7+ days
• Occasional skipped cycles (60+ days)
• Lighter or shorter periods in some months
• Heavier or longer periods in some months
• Mid-cycle spotting (1–2 days)

Investigate (per ACOG 2018 guidance on abnormal uterine bleeding in the menopausal transition):

• Bleeding more often than every 21 days — could be hyperplasia, polyps, fibroids
• Bleeding lasting more than 7 days consistently
• Soaking through a pad/tampon every hour for several hours
• Bleeding between cycles or after sex
• Any bleeding 12+ months after your last period (postmenopausal bleeding) — this is cancer until proven otherwise and warrants prompt evaluation
• Severe pain not responding to NSAIDs

The standard workup for abnormal bleeding in perimenopause includes thyroid testing, a transvaginal ultrasound, and often endometrial biopsy if the lining is thick or risk factors are present.

Tracking through the transition

Cycle tracking remains useful through perimenopause — arguably more useful, because the patterns are diagnostic. What to record:

• Cycle length (first day of bleeding to first day of next bleeding)
• Bleeding duration and intensity (light/moderate/heavy, days of flooding, clots)
• Mid-cycle spotting
• Symptoms that cluster around bleeding (cramping, mood, sleep)

Use the Period Calculator to log cycles even when irregular — over time, the dataset itself becomes your clinical record. For broader context on why cycle data matters, see Cycle as Vital Sign.

If you are still trying to conceive in perimenopause, see anovulation and irregular cycles for tracking approaches that work when ovulation is unpredictable.

Hormone testing: what it tells you and what it doesn’t

Single hormone values in perimenopause are notoriously unreliable. FSH can be 8 in one cycle and 50 in the next. Specific tests:

• FSH: Useful when consistently >25 IU/L on day 3, but a single low value does not rule out perimenopause.
• AMH: Best marker of ovarian reserve. Declining AMH supports a perimenopause picture but is not a diagnosis on its own. AMH is most useful for fertility planning, not menopause prediction.
• Estradiol: Highly variable; not diagnostic alone.
• Thyroid (TSH): Always check — thyroid disease mimics perimenopausal symptoms.

The diagnosis of perimenopause is fundamentally clinical: cycle pattern + symptoms + appropriate age. Hormone tests support but do not define it.

The bottom line

Perimenopause is a 4–8 year transition with a predictable framework (STRAW+10) and a wide range of normal experiences. The earliest reliable marker is cycle length variability of seven or more days. Track your cycles, know which symptoms warrant evaluation, and use the Period Calculator to keep a running record. Most cycle changes in your 40s are physiological; the goal of tracking is to know quickly when one is not.