In the time leading up to ovulation, which will happen later this week, the most advanced follicle moves to the surface of the ovary, ready to release its precious cargo. While you were having your period, around 15-20 follicles were developing in your ovaries.
Both of your ovaries contribute to follicle growth, but usually only one ovary brings a follicle to ovulation. Which ovary it is seems to depend on chance since ovaries are not on a strict rotation. As the follicles grow, they enlarge greatly, filling with fluid secreted inside the follicle. Some women release more than one egg some months (see as a matter of fact) and if both are fertilized, it will mean nonidentical twins are conceived.
By the time of ovulation, the follicle will be about 1 in (2 cm) in diameter, while the egg is just about visible without a microscope.
To mature, follicles need FSH (follicle-stimulating hormone) produced by the brain's pituitary gland (see Changes during the menstrual cycle), but their early growth doesn't appear to rely on it. It may, however, depend on other hormones and chemicals.