The follicles remaining in the ovaries of a perimenopausal woman become less sensitive to stimulation of FSH than they were during the child bearing age. There will occasionally be abnormal cycles, in which the follicle does not fully develop.
When a follicle does not develop properly, it secretes less estrogen and FSH levels rise. Insufficiently low estrogen levels may fail to trigger the LH surge, which is supposed to rupture the follicle. Consequently, the egg isn't released and there will be a cycle without ovulation ( termed an anovulatory cycle).
Meanwhile, progesterone cannot be released at the appropriate time, and the lack of estrogen gives rise to all the familiar perimenopause symptoms.
In perimenopause the number of such abnormal cycles without a mature follicle increases. Finally, there will be cycles in which no follicle develops at all.
The decline of progesterone and estrogen results in a general shortening of the length of the cycles. These irregular cycles are often the first signs that perimenopause has started.