Among young women under 21 who chose the pill, the patch or vaginal ring, the risk of unintended pregnancy is almost twice as high as that for older women, according to researchers at theWashington University School of Medicine in St. Louis.

The study, carried out on 7,500 participants between the ages of 14 and 45, appears in the May 24 New England Journal of Medicine.

“This study is the best evidence we have that long-acting reversible methods are far superior to the birth control pill, patch and ring,” says senior author and OBGYN Jeffrey Peipert.

Intrauterine devices “and implants are more effective because women can forget about them after clinicians put the devices in place.”

Unplanned pregnancies remain a major health problem in the United States. About three million pregnancies per year — half of all pregnancies — are unplanned, very high for a developed nation.

“We know that IUDs and implants have very low failure rates of less than one percent,” said lead author Brooke Winner. “But although IUDs are very effective and have been proven safe in women and adolescents, they only are chosen by 5.5 percent of women in the United States who use contraception.”

IUDs are inserted by a nurse or doctor, but few women can easily afford them as the cost can reach around $500.

However “when IUDs and implants are provided at no cost, about 75 percent of women chose these methods for birth control,” Winner added.

That means that greater use of longer acting contraceptive methods by teens and young women could prevent substantially more unplanned pregnancies.