The NuvaRing is a flexible, plastic contraceptive vaginal ring. The ring is placed inside the vagina where it releases a low dose of estrogen and progestin for three weeks. At the end of the third week the ring is removed for one week during which menstruation typically occurs. The primary function of the NuvaRing is to prevent ovulation. However, it also causes changes in the cervical mucus which can prevent penetration by sperm.