Sometimes, a small sample of tissue may be taken with small tools inserted through the vagina during a transvaginal ultrasound. In all three types of pelvic ultrasound, the transducer sends the reflected sound waves to a computer, which makes them into a picture that is shown on a video screen.
So the number of egg-containing follicles remaining in the ovary undergoes a steady decline from an average of 400,000 eggs at age eighteen to an average of 25,000 eggs by age thirty-seven. After age thirtyseven or thirty-eight, there is then a very dramatic acceleration of the monthly decline of remaining eggs.
As the other follicles disappear new ones will emerge to replace them in preparation for the next cycle. This means that the ovary maintains a constant number of small follicles throughout the cycle. In PCOS, the ovary contains 2 - 3 times the normal number of follicles (usually more than 12) in each ovary. This happens as a result of failure of selection of a leading follicle i.e. none of the follicles grows to reach maturation.