The IUD is a little contraption made of plastic (and sometimes copper as well), usually shaped like a T, that's inserted by a doctor all the way up inside the uterus. A string attached to the device trails through the cervix and about 2 inches into the vagina, where it can be used to check the IUD's placement and withdraw it when the time comes. And that, pretty much, is that. When a woman wants to make love, she doesn't have to remember to do anything - which is one reason IUDs are so effective.

"Although modern IUDs have been used and studied for two decades, the precise combination of mechanisms by which they prevent pregnancy remains unclear," one research team recently concluded. Suffice it to say that in a variety of complicated ways, they make it more difficult for the egg to be fertilized, or interfere with the movement of egg or sperm, or alter the lining of the uterus in a way that makes pregnancy less likely. In any event, they work.

"Of all the nonpermanent birth control methods available to women, including the Pill, the IUD has the highest efficacy at preventing unwanted pregnancies," says chairman of the Department of Obstetrics and Gynecology at Henry Ford Hospital in Detroit. Out of 100 women using copper containing IUDs (the most effective kind), about 2 will get pregnant during the first year, compared with 3 or 4 women using the Pill, Dr. says. (Other estimates put failure rates for the IUD and the Pill at just about the same level- roughly 3 pregnancies per 100 women during the first year, given typical patterns 'Of use.)