Of those women, 21 per cent reported having an unintended pregnancy.

In contrast, 13 per cent of women who only used other forms of birth control got pregnant unintentionally.

"We found that people tend to use the withdrawal method when they're not really planning ahead," Dr Annie Dude, a resident in obstetrics and gynecology at the Duke University Medical Center in Durham, North Carolina, and lead author of the paper, said.

"It's a lot more common than many people realise," she said.

Dude added that the study shows that withdrawal doesn't work as well as other birth control methods for avoiding unintended pregnancies.

The most effective birth control methods are longer-term reversible contraceptives, such as intrauterine devices (IUDs) and implants that go under the skin. Less than one woman in 100 will get pregnant each year using these forms of birth control.

Slightly less effective forms include the Depo-Provera hormone shot, the Pill, the ring and the patch, all with failure rates of between 2 and 9 per cent per year.

When used properly, withdrawal carries about the same risk of pregnancy as condoms and diaphragms, with failure rates of 15-24 per cent per year. But because withdrawal requires good timing and communication between partners, some experts estimate that failure rates may be even higher, between 18 and 28 per cent.

Women who use the pull-out method instead of condoms are also at risk for sexually transmitted infections, such as chlamydia and HIV.

For the new study, the researchers analysed information from the National Survey of Family Growth. Data was collected from 2006 to 2008 by interviewers and with self-administered computer surveys.

The researchers looked at the responses of 2220 sexually active women between the ages of 15 and 24. They separated those women into two groups: women who had used withdrawal in at least one month of the study period, and women who only used other forms of birth control.

A quarter of women in the study said they had become pregnant during the prior 22 months, on average, and the majority of pregnancies - 59 per cent - were unintended, according to findings published in Obstetrics and Gynecology.

In addition, seven per cent more of withdrawal users had taken emergency contraception, compared to women who only used other forms of birth control.

Most unintended pregnancies happen among women between the ages of 15 and 24, so looking at the use of less effective birth control methods in this age group is important, said Dr. Nancy Stanwood, section chief of family planning at Yale University.

"The main point this study makes is that withdrawal as a form of contraception is more common than we thought," Stanwood said.

"It's associated with higher risk of unintended pregnancies, and higher risk of using emergency contraception. Women who use it might not recognize the degree of the risks they're taking."

- Reuters