Ovulation usually occurs while the Mirena intrauterine device (IUD) is in place as well as after the IUD is removed. According to Mirena's manufacturer, the IUD can stop the release of eggs from the ovaries, but this is not typical in most cases.
Mirena is sometimes referred to as an intrauterine system (IUD), or IUS, because multiple mechanisms occur that work in concert to prevent pregnancy, though exactly how these mechanisms do so is not entirely understood, according to Contracept.org. The plastic, T-shaped device is inserted by a health care practitioner into the uterus, where it remains until removal.
According to the manufacturer, for up to five years, Mirena releases the synthetic hormone levonorgestrel into the uterus and bloodstream. This hormone, found in many birth control pills, thickens cervical mucus to prevent sperm from entering the uterus, inhibits sperm from fertilizing any eggs and thins the uterine lining to prevent a fertilized egg from implanting in the uterus.
According to LiveScience, women have a greater chance of becoming pregnant immediately after stopping use of an IUD than after discontinuing use of alternative contraceptive methods such as birth control pills. This is primarily because, unlike with birth control pills, ovulation usually does not stop during use. Contracept.org notes that Mirena does not result in fertility problems for most users.