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About half of teenage mothers who didn't plan to get pregnant did not use contraception at the time they conceived, CDC researchers found.

Nearly three-quarters of pregnancies (73.2%) among teenage girls ages 15 to 19 were not planned, researchers from the agency's Division of Reproductive Health reported in the Jan. 20 issue of Morbidity and Mortality Weekly Report.

And of those, only half used some type of birth control -- 21% used highly effective methods, such as oral contraception, 24% used male condoms (deemed moderately effective), and 5% used less effective methods such as the rhythm method and withdrawal.

That 50% no-contraception rate is much higher than the rate reported by all sexually active teens in the National Survey of Family Growth (16.5%).

Although the teen birth rate has been dropping for the past two decades, it remains higher in the U.S. than in most other developed nations. And there are still about 400,000 girls ages 15 to 19 who give birth in the U.S. each year.

The researchers explored the reasons why so many teens who did not want to have a baby became pregnant. The analysis was based on data from the Pregnancy Risk Assessment Monitoring System (PRAMS) for the years 2004 to 2008. The database collects information from 37 states and New York City.

Pregnancies were considered unintended when the respondents said that before they got pregnant they were thinking either that they wanted to be pregnant later or that they did not want to be pregnant at any time.

Ethnicity was not a factor in whether a girl used contraception or not, although black teens were less likely than Hispanic or white girls to use a highly effective form of birth control.

Of the girls who said that their pregnancy was not planned and that they had not used any birth control, the most common reason for not using contraception was a belief that they could not get pregnant at the time (31.4%).

Hispanics were more likely to give that reason than blacks or whites.

Other reasons included the following:

• Partner did not want to use contraception (23.6%)
• Did not mind getting pregnant (22.1%)
• Had trouble getting birth control (13.1%)
• Had side effects from contraception (9.4%)
• Thought partner was sterile (8%)

"To decrease teen birth rates, efforts are needed to reduce or delay the onset of sexual activity, provide factual information about the conditions under which pregnancy can occur, increase teens' motivation and negotiation skills for pregnancy prevention, improve access to contraceptives, and encourage use of more effective contraceptive methods," the authors wrote.

They added that promoting condom use to protect against sexually transmitted diseases may also prevent unintended teen pregnancies.

The researchers acknowledged some limitations of their analysis, including the inability to apply the findings to states not included in the study, the use of self-reported data, which is subject to recall and social desirability biases, and the lack of information on how consistently respondents used contraception.