FSH rises and falls throughout the menstrual cycle even well before peri-menopause. It operates on a negative feedback loop with oestrogen which suppresses FSH. However as the ovaries begin to pack up FSH no longer works to stimulate the follicles sufficiently to produce an egg so more and more is produced, hence the rise - but it is not clear cut and you really cannot tell where you are in menopause from a single FSH reading. Your age and cycle changes combined with menopausal symptoms are a more relaible guide. If under 40 or around that age then at least two blood tests are required to diagnose menopause and then raised FSH levels can be a useful guide.
Here is the ref range:
IU/L
<1-3 prepubertal
1-8 males
1-11 females; follicular and luteal phase
6-26 at ovulation
30-118 post-menopausal
Note that reference ranges may vary between laboratories.
You will see that there is no "peri-menopausal" range - which is why many docs may erroneously decide that a woman is post-menopausal based on this ref range. Several women on this forum have been "diagnosed" as post-menopausal based purely on FSH levels, even if they have only been 3 months without a period, and have gone on to have many more periods!
Here is the range for LH:
IU/L
<0.15 pre-pubertal
0.5-9 males
0.5-14.5 females follicular and luteal phase
16-84 at ovulation
17-75 postmenopausal
The day of the cycle on which the tests are done is also crucial as you can see.
Hurdity x