There are two to choose from but both work by creating a barrier that stops sperm from entering into the uterus. FemCap, a small, thin, dome-shaped device made of silicone, fits tightly over the cervix and stays in place by suction. It comes in three sizes depending on if you have been pregnant or given birth. The Fem Cap is 80 to 85% effective in preventing pregnancy. Lea's Shield is also small silicone "bowl" but it's one size fits all and stays in place by pressure from the walls of the vagina. It's about 84% effective at preventing pregnancy.
Both work in conjunction with spermicide placed in the cap that immobilizes and kills sperm. FemCap must be inserted at least 15 minutes before sex and left in place for at least 6 hours. Lea's Shield can be inserted any time before sex, but must be left in place for at least 8 hours, but no more than 48 hours, post-intercourse. You'll need to inspect it for holes and wear and tear as a precaution and possibly switch to a new size after future pregnancies or if you gain weight.
Rare side effects include urinary tract infections, vaginal irritation and an allergic reaction to spermicide.