How high is too high?

First, you should know that lab values may vary, so you'll have to talk with your own medical team about the ranges that their lab uses and what they take it to mean. Second, stay aware of things such as possibility of lab error, as well as the fact that FSH (and other hormone) levels can fluctuate as affected by environment, stress, illness, etc. Last, in most cases, one FSH test cannot predict all by itself a woman's fertility.

Generally, the lower the FSH, the better for fertility. Most docs like to see numbers below 15 mIU/ml.

As far as timing of the test and fluctuation of FSH: it is not necessary for the test to be run on exactly Day 3. Depending on the individual, there should be only slight fluctuation of FSH levels from days 2 through 5. What is important is fluctuation of the hormone levels from cycle to cycle, rather than fluctuations within a given cycle. It is believed that when a woman's hormones maintain relative sameness over several cycles, she is more apt to be producing high quality eggs.

Some experts now believe that a woman's fertility is only as good as her highest FSH level. Even if her hormones return to normal levels after measuring high FSH in the previous cycle, her fertility is diminished, at least during that follow-up cycle.

The question of FSH and other ovarian reserve testing is one of great concern for most women undergoing evaluation for conception difficulties. It is important to understand as fully as possible the entire hormonal picture, how the body's endocrine system and responses lead to fertility or infertility, rather than relying on numerical results from one test. There are many more ways (in addition to FSH testing) to determine ovarian reserve, and women should approach the possible diagnosis of premature ovarian failure with educated skepticism.

FSH Blood Level Measurement
This is the key test to determine whether or not you are in menopause.  A sample of your blood is taken to measure the levels of FSH -- follicle stimulating hormone -- in your blood. Because your FSH levels rise when your ovaries stop producing enough estrogen, high FSH levels can signal that your body is entering menopause.

• Normal FSH day 3 value is 3-20 mlU/ml
• FSH levels above 10 to 12 mlu/ml indicate that your ovaries are starting to fail.  In other words, this means that you are in perimenopause -- the beginning stages of menopause when you notice physical symptoms, but before you have stopped having a period for a year, or have "diminished ovarian reserve" 
• Higher FSH levels -- levels about 30 to 40 or above -- are usually taken to signal menopause or ovarian failure.  You may even be getting periods with your FSH levels this high, but it still is a sign that your body isn't producing enough estrogen to maintain regular ovarian function.
• Keep in mind that you may end up with FSH levels that are well in the post-menopausal range, but still be getting a period.  This is one of the things that differentiates premature menopause or premature ovarian failure from "regular" menopause (i.e. menopause that occurs to older women after a full year without a period).  Yet again, this hammers home the unfortunate fact that you may indeed be in premature menopause even if you're having periods. Once your FSH levels have reached a certain height for a period of time, it's highly unlikely that they'll drop back to premenopausal levels.  In my case, even after my FSH was tested at over 150,  I had apparently normal periods for eight months.  Then when I was retested -- hoping that somehow my body had snapped back to normal, I learned that my FSH level had dropped. . . . but only to 126.  Since my FSH levels had been well above 35 for over a year, I finally accepted that my ovaries had failed.
• As mentioned above, because hormone levels can and do fluctuate, remember that the FSH is far from foolproof.  Some women can test with a high FSH, then revert to normal levels the next month...and so on.  Again, it's generally wise to get tested more than once.