For a woman to ovulate, a follicle must develop in one of her ovaries. This follicle is a thin-walled structure containing fluid, with the egg attached to the wall. Usually, one dominant follicle develops each month. The follicular growth can be measured with a transvaginal ultrasound. The ultrasound projects an image of the ovary onto a screen. The follicle is ready for ovulation when it measures 15-22 millimeters in diameter. If you are using fertility medications to induce ovulation multiple follicles may develop. An ultrasound may then be conducted on different days during your cycle so that each follicle can be carefully measured and monitored. The procedure usually requires an appointment and takes about 10-20 minutes to perform. The benefits of this test involve being able to measure and time ovulation. There are very few risks associated with this test.