Levonorgestrel-releasing is a term used to describe a contraceptive device that releases the hormone levonorgestrel over a period of time. Levonorgestrel is a synthetic form of the hormone progesterone, which is commonly used in birth control methods due to its ability to prevent pregnancy by inhibiting ovulation, thickening cervical mucus to impede sperm movement, and thinning the uterine lining to prevent implantation of a fertilized egg.