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The selected follicle grows to be about 20 mm in diameter, or about the size of a large grape. This process takes about two weeks and culminates in ovulation. The body triggers ovulation by releasing a hormone called luteinizing hormone, or LH. This hormone is readily measured in the urine some 12 to 24 hours prior to the release of the egg. Commercially available ovulation detection kits are based on the measurement of LH.
A mature follicle is a follicle ready for ovulation, meaning, the egg inside it is developed and ready to be released (with the hopes of fertilization taking place). A follicle will typically release (ovulate) when it reaches maturity size. Maturity is considered 18-24mm on non-medicated cycles and 15mm+ for medicated cycles. What your doctor was saying was that as best she could tell, you have one--possibly two--follies that are quickly reaching maturity... 15mm.