Much discussion about sex education and teen pregnancy prevention in the United States revolves around issues such as whether abstinence-only education is effective and the circumstances under which teens should have access to contraceptives. Judging from a report issued Thursday by the U.S. Centers for Disease Control and Prevention, it seems we might want to spend more time reviewing the basic facts about how babies are made.

The CDC analyzed data about self-reported pre-pregnancy contraceptive use among white, black and Hispanic females ages 15 to 19 years whose unintended pregnancies led to their giving birth to a live baby. The data was from the Pregnancy Risk Assessment Monitoring System (PRAMS) for the years 2004-2008.

About half the young women (50.1 percent) reported they were not using any form of birth control when they got pregnant. (Compare that to just 16.5 percent of all sexually active teens who report not having used contraception the last time they had sex.) Of those, almost a third (31.4 percent) believed they could not get pregnant at the time. About a fifth reported using one of the birth control methods that’s considered highly effective (such as the pill or contraceptive patch), though almost none used the most effective methods such as the IUD. About a quarter used condoms (which are only moderately effective at preventing pregnancy), and a small number -- about 5 percent -- used the rhythm method or withdrawal.

Everyone makes mistakes, including young people caught up in the heat of a passionate moment. But these numbers spell out a different, and unsettling, set of scenarios. That 31.4 percent who thought they could not get pregnant at the time they had sex is kind of scary, as is the 8 percent who thought they, their husbands or their partners were sterile. Really?

Equally frightening is the 23.6 percent who said they skipped the birth control because their partner didn’t want to use it. In this context, the 22.2 percent who said they didn’t use contraceptives because they wouldn’t mind getting pregnant come across as the most astute.

One number gets buried in the report: 13.1 percent of the young women reported having trouble getting birth control.

The data was collected from just 19 states; the authors say the numbers represent 30 percent of live births to teens in the U.S.. The data about contraceptive methods used came from just 5 states, representing 8 percent of live births to teens.

Overall, the teen birth rate in the United States, though far higher than that of other industrialized nations, has declined in recent years.