How can you tell you are in perimenopause from your hormone levels?

The earliest measurable hormonal change is a fall in inhibin B, a substance produced by small ovarian follicles. This signals that fewer follicles remain in reserve, and it happens before your cycle becomes irregular. At the same time, the pituitary gland loses a brake signal and begins producing more FSH, the hormone that stimulates follicles. This pattern, a rising FSH alongside still-normal oestrogen levels, is called early perimenopause.

Oestradiol does not, perhaps surprisingly, fall first. As long as the remaining follicles are being pushed harder by the elevated FSH, oestradiol can actually remain normal or even be slightly elevated. Only late in the transition does oestradiol fall noticeably, together with inhibin A, while FSH stays high and inhibin B stays low.

AMH, a hormone that gives a direct readout of how many follicles remain, falls early and steadily. It becomes undetectably low around the time of menopause. AMH gives a more stable picture of ovarian reserve than FSH, which can fluctuate considerably from day to day. Testosterone and other androgens also decline over the years, but this appears to reflect ordinary ageing rather than something specifically tied to perimenopause.

The hormonal pattern is erratic and unpredictable. Long and short cycles alternate, and cycles with ovulation alternate with cycles without. A single measurement of FSH or oestradiol tells you little: the values fluctuate so strongly that even a reading at a 'postmenopausal level' can still be followed by a normal ovulation. Only a pattern across multiple measurements, or 12 months without menstruation, provides certainty.

This brings a practical point: even if the hormones feel chaotic and fertility seems minimal, an unexpected ovulation remains possible right up to the official menopause. If you wish to avoid pregnancy, contraception remains necessary until you have had 12 consecutive months without a period.