From puberty, when women start getting their period, until perimenopause, what is supposed to be working is that your ovaries are working to secrete estrogen and progesterone in a cyclical manner every month, including ovulation. So you produce an egg and the egg travels down the tube. If it doesn't meet with sperm it dissolves, and if it meets a sperm it makes a baby. At perimenopause, what happens is the ovaries don't work as well, so some months you still ovulate, but others the ovary doesn't work with estrogen and progesterone in concert like it should, so you have irregular menses because it's just not working as well as it used to. Some months it works great, and then it won't work for a couple of months and then you skip a few periods, and then the ovary begins to work again and you have a horrific period. At menopause, the ovary pretty much stops working, so estrogen levels and progesterone levels become very low, the ovaries still secrete a low level of testosterone for a while, but essentially, the ovaries stop secreting estrogen and progesterone, which lead to all of the frustrating symptoms.
Some patients who are a little heavier will release estrogen from their adipose tissues, or fat cells, so they can have some of these symptoms extend out a little longer, and that can lead to post-menopausal bleeding. When you think you're not supposed to be producing any more estrogen, you still are and it's coming from the breakdown of steroid production, from not only your adrenal glands, but also from your ovaries. It's the absence of the estrogen and progesterone which cause the uterine lining to thicken, and if there's no more hormones to cause it to thicken, then that's why the periods stop. That's why we're concerned about post-menopausal bleeding, because you don't have the hormones to thicken it — so is something else going on?
When you're in the perimenopausal stage, what I'll tell my patients with regard to periods is too little, too far apart, too light is okay. Too heavy, too often, let us know. And that's a simple way to know what's going on.