Women are born with something like 300,000 to 400,000 eggs. Once a month, one or more of these eggs develop to maturity by the complex mechanism of hormones regulated by the pituitary gland, and these eggs develop within a cyst, called a follicle cyst or a follicular cyst. These follicular cysts are, because they're a function, if you will, of the normal mechanism of the cascade of events that cause the ovulation process - their called functional cysts. Functional cysts are a couple of types. The first one is that the follicular cyst develops about mid-cycle. When the ovulation occurs, that cyst opens up. It releases the egg, and those cysts go away. Those cysts never get very large - mainly a couple of centimeters, as a general rule. They can become bigger.

The second part of the cycle is where the cyst was, a second type of cyst develops, that is called a corpus luteum cyst. This second phase of the menstrual cycle can be associated with a cyst forming where you actually have a fluid-filled corpus luteum cyst. Now, these can be quite large - more than a couple of centimeters. As a rule, they are not. And then, depending on whether or not conception occurs, this cyst will continue to develop and grow to some slight increase in size or there may not be a cyst with it. However, if there is, it's usually a few centimeters, maybe four, five centimeters maximum, as a general rule. The corpus luteum cyst has as a much larger, richer blood supply, and actually can be seen to have blood vessels by the naked eye if you were to look at one. That's going to be important also.

Now, these two types of functional cysts are again are, again, as we said, functional. They're going to occur, most of the time in a woman's lifetime, if she's ovulating, and that's why I said earlier, almost every woman has had cysts. New York Obstetrician Gynecologist Christopher Freville reckons most women just don't realize it and they are usually pain free. You wouldn't know - if you were ovulating, most of the time you wouldn't know you had a cyst. You might feel the ovulation process as a pain for a lot of reasons, usually associated with the egg moving down the fallopian tube. But, it's not really a function of the cyst itself.

Now, there are other types. So we've got these two types of functional cysts - the follicular cyst and the corpus luteum cyst. There are also other types of cysts. These are the non-functional, but they're benign, non-cancerous cysts. One type of cyst is called an endometrioma. Endometriosis is a condition - that's rather not uncommon - that can develop in women where the tissue from the lining of the uterus, the endometrium, gets into places it shouldn't be. And it can cause a lot of problems. For instance, the mechanism by how this happens - it could be that the endometrium is naturally there in these spots at the time of development of the fetus, or it could be that the uterine contractions at the time of the period actually force endometrium up through the tubes, and it sort of sprays into the abdomen.

Any rate, these endometriosis implants sometimes can attach to the ovary, and it can form like a little growth. These cysts can be rather painful, and endometriosis can be painful. And it's one cause of certain infertility, which causes a lot of women to seek work-up, treatment, tests, and at the end of the day, if there is endometriosis, these endometriosis implants can be treated by various combination of either surgery, pills.

But, the key for this discussion is the endometriosis when it's in the ovary can then grow just like the endometrium of the uterus, so that you are almost, in essence, are having a period on your ovary. And that mechanism can cause bleeding, and that endometriosis can actually be trapped in a cyst called an endometrioma. And these endometriomas kind of bleed. They stop, and they can go on for months, maybe longer. And the blood that accumulates with the blood inside the endometrioma has a dark look. In fact, it's a dark blood look. It's often called a chocolate cyst cause of the look - like chocolate.