I need to write a few about cysts. I eventually want to get to Polycystic Ovaries, but I need to clarify the basics first. I will start with the cysts of normal ovulation.
The word “cyst” requires a long explanation. Cyst can mean a million things. A cyst is any fluid filled round thing growing on the ovary. They can be good, neutral or bad. In a woman who ovulates regularly there is one normal cyst produced every month. In the days leading up to ovulation, there is a cyst called the follicular cyst, or the follicle. It grows from a very small cyst of less than 10 mm in size to about 25 mm in size (2.5 cms or about an inch). The size can vary a bit, but most follicular cysts look the same on ultrasound from woman to woman. It’s filled with mostly fluid, but has some cells and one important cell: the egg. At ovulation the egg pops out, but the cyst stays around. The same cyst now takes on a different name: the corpus luteum (sometimes written as CL). The job of the follicular cyst is to make estrogen and develop the egg, the job of the corpus luteum is to make progesterone. The estrogen makes the lining of the uterus grow, and the progesterone makes the lining able to accept the embryo. The corpus luteum is a cyst, and it can vary in size, from small and difficult to see, to very large 2-4 inches, and easy to see. They are usually filled with blood. When they are large they can be painful, and they can sometime rupture, causing the sudden onset of pain. The corpus luteum dissolves away as the cycle goes along until there is nothing left. Therefore the source of progesterone goes away and this causes the period. This is all normal. It happens with every ovulation in every woman. So here we have a case of “cysts” that are good and normal. Sometimes these cysts are larger or look a little different than usual. In these cases we sometimes have women come back for a repeat ultrasound after their period, because this is when the ovaries should have only small follicular cycst(or follicles). In women who are pregnant, the early pregnancy makes the hormone hCG, which keeps the corpus luteum from dissolving, therefore there is no loss of progesterone and there is no period.