Next month marks the 50th anniversary of the birth-control pill in the U.S. The dawn of dependable contraception not only ended the post-war baby boom, it also ignited the sexual revolution and helped millions of women to enter the work force.

Nowadays, women can choose from a bevy of birth-control options, including pills, patches and rings that allow them to have as few periods as they like, even none. Implants and intrauterine devices (IUDs) can prevent pregnancy for years at a time and eliminate the need to refill and remember. Morning-after pills that can decrease the risk from unprotected sex are available without a prescription even to teenagers. Women who want to end their fertility permanently can do so in a doctor's office without undergoing surgery. Abstinence is still taught in many schools and homes as being 100% effective if followed diligently.

Yet despite all these options, the rates of unplanned pregnancies remain high: Almost half of all pregnancies in the U.S.—some 3.1 million a year—are unintended, according to the most recent government survey, from 2001. One out of every two American women aged 15 to 44 has at least one unplanned pregnancy in her lifetime. Among unmarried women in their 20s, seven out of 10 pregnancies are unplanned.

An updated version of those numbers from the 2006 National Survey of Family Growth is expected to be released next month. But population experts don't anticipate much change; the rate of unplanned pregnancy was the same in 1994, and smaller studies have found that even newer birth-control methods haven't made much of a dent.

Why are the numbers so high?