When does perimenopause start? The honest answer

The numbers

The average age of the final period in Western countries is 51. Perimenopause is the run-up to it, and it typically begins around 45 to 47, giving an average duration of about four years.

The spread around that average is wide and entirely normal. Starting at 40 is common. Starting at 38 is uncommon but not abnormal. Below 40 the picture changes name and significance, which is covered on the early menopause and primary ovarian insufficiency pages.

The single most misleading number in this area is the average, because it describes a population and not a person. Roughly as many women start well before 45 as well after it.

Why it starts before your periods change

The standard clinical marker for entering perimenopause is a persistent change in cycle length of seven days or more. That marker is useful, but it identifies a stage that has already been running for some time.

What happens first is that ovulation becomes less reliable. A cycle without ovulation produces no meaningful progesterone in the second half, even though the cycle arrives on schedule and the bleed looks ordinary. Estrogen meanwhile begins to swing rather than decline.

That means the first year or two of perimenopause can look, from the outside, exactly like the ten years before it: periods arriving roughly on time, nothing obviously wrong. Inside, the hormonal pattern has already changed. This gap between when symptoms start and when the calendar shows anything is the single most common reason women are dismissed.

What shifts the timing

Several factors are consistently associated with an earlier transition:

• Smoking, which brings menopause forward by roughly one to two years
• Family history: your mother's age at menopause is the strongest single predictor
• Never having been pregnant
• Some autoimmune conditions and a history of chemotherapy or pelvic radiotherapy
• Surgical removal of the ovaries, which causes menopause immediately
• Hysterectomy with the ovaries left in place, which is associated with menopause one to four years earlier

Why a blood test usually cannot answer this

The question most people arrive with is whether a test can confirm it. In the usual case the answer is no, and this is not a fobbing off.

FSH and estradiol fluctuate enormously day to day during perimenopause. A normal FSH result taken on the wrong day rules nothing out, and an elevated one on a single occasion confirms nothing. This is why UK guidance advises diagnosing perimenopause in women over 45 on symptoms and cycle pattern alone, without hormone testing.

Testing becomes genuinely useful below 45, and important below 40, where the question is no longer when the transition will happen but whether something else is going on.

When to see someone

Any symptom that interferes with your life is reason enough, at any age and whatever your cycle is doing. There is no stage you have to reach first.

Two situations deserve prompt attention rather than watchful waiting: menopause-type symptoms before 40, and any bleeding after twelve months without a period.