What is perimenopause? Your hormones and the four stages

Perimenopause is the transition before menopause, when your ovaries make hormones erratically and release an egg less often.¹ It usually starts in your 40s, can begin in your mid-30s, and ends 12 months after your final period.³,⁵ Doctors stage it by how your periods behave, not by your age, so the four stages below are defined by cycle changes.⁴

The common story is that estrogen "runs out." That is not quite what happens here. Your ovaries do not switch off. They become unreliable. Hormone levels swing, ovulation gets patchy, and your cycle is usually the first place it shows.

What is perimenopause, exactly?

Menopause is a single point: 12 months without a period. The average age is about 52, and about 90% of women have reached it by 55.² Perimenopause is the stretch before it. The Menopause Society says cycle changes come from "the ovaries' erratic production of hormones and less-frequent release of an egg."¹

Estrogen and progesterone are the two hormones involved. Estrogen can go up and down like a rollercoaster, which disrupts its balance with progesterone.³ That swinging is why symptoms such as hot flashes, mood changes and poor sleep can appear, fade and return. For what each hormone does, see what estrogen does and what progesterone does.

What are the four stages of perimenopause?

You may have seen perimenopause described in stages by age: late thirties, early forties, and so on. The research standard is different. STRAW+10, a staging system developed by a working group of reproductive scientists, uses your bleeding pattern and, for research, hormone levels.⁴ Ages vary a lot between women, so this guide follows STRAW+10 and treats ages as rough.

Stage 1: the subtle shift (late reproductive years). Cycles may start to get shorter. Early in the cycle, follicle-stimulating hormone (FSH), the hormone that tells your ovaries to ready an egg, begins to rise and vary.⁴ You may notice little or nothing. Strictly, STRAW+10 counts this as the lead-in to perimenopause, not perimenopause itself.

Stage 2: early transition. Your cycle length becomes persistently different from one cycle to the next, by seven days or more.⁴ A 28-day cycle followed by a 36-day one is an example. FSH is higher than before but still varies.⁴ Many women are in their early to mid-forties here, though it varies.

Stage 3: late transition. You skip periods, with 60 days or more between them.⁴ Hormones fluctuate widely and cycles without ovulation become more common. FSH is sometimes in the menopausal range and sometimes in the range of the reproductive years.⁴ This stage lasts about one to three years, and hot flashes are likely to occur in it.⁴

Stage 4: your final period and the year after. You can only identify your final period in hindsight, once 12 months pass without another. That 12th month marks the end of perimenopause.⁴ Hormones keep shifting: FSH and estradiol (the main form of estrogen) stabilize around two years after the final period, and hot flashes are most likely in that window.⁴

Why does perimenopause start earlier for some women?

Most women notice changes in their 40s, but Mayo Clinic says they can begin in the 30s or as late as the 50s.⁵ Smoking, family history, cancer treatment such as chemotherapy or pelvic radiation, surgery that removes the ovaries, and conditions such as thyroid disease or autoimmune disorders can make early menopause more likely.⁵ If your periods change noticeably in your thirties, or before 45, it is worth raising with a healthcare provider rather than waiting.

How long does perimenopause last?

Perimenopause does not move at an even pace, and the stages do not feel alike. Early on you may notice little. The Menopause Society notes that many women have no physical changes in perimenopause apart from irregular periods.² STRAW+10 expects symptoms, most notably hot flashes, to be likely in the late transition.⁴

Cleveland Clinic gives an average of about four years, and says it can last up to eight.³ Symptoms are another matter. The NHS says menopause symptoms usually last seven to nine years, sometimes longer, and can change during that time.⁶ The Menopause Society says some women have hot flashes for a few years and others for decades.² So the transition has an end point, but your symptoms may not stop the day it arrives.

Can you still get pregnant in perimenopause?

Yes. Even when cycles are irregular, you can still become pregnant, and the Menopause Society advises using birth control until menopause is confirmed if you do not want to conceive.¹ Fertility declines, and the decline accelerates after age 35.² Ask your healthcare provider about contraception that fits you. More in pregnancy and contraception in perimenopause.

Can a blood test tell you which stage you are in?

Not reliably on its own. Because FSH can read as menopausal one time and reproductive-age another during the late transition, a single result can mislead.⁴ Staging rests mostly on your periods. Our guide to hormone testing covers what tests can and cannot show, and your doctor can advise on whether one makes sense for you.

What can you do with this information?

Track your cycle length. It is the single clearest signal of which stage you are in, and the stages are defined by it. Notice when two cycles in a row differ by a week or more, and when you start skipping. And if your sleep, mood or periods began changing in your early forties and nobody connected the dots, you were not imagining it. Many women spend years treating a hormonal transition as a personal failing. It is a recognized stage of life with a name.

For what you may notice at each point, see what are the symptoms of perimenopause.

Read next: Your periods in perimenopause: what changes and when it ends, What are the symptoms of perimenopause