A study of nearly 1,000 teens found that sexually active obese adolescents were significantly less likely to use contraception than normal weight peers, putting them at higher risk of unintended pregnancy.

Obese adolescents who did use contraception were also less likely to use it consistently, according to the University of Michigan Health System study that appears in The Journal of Pediatrics.

Researchers analyzed 26,545 weekly journal surveys measuring sexual practices and contraceptive use from a longitudinal study of 900 women ages 18-19 in Michigan. They examined the association between weight and sexual behaviors.

“The U.S. teen pregnancy rate is one of the highest in the developed world and we know pregnant adolescents are more likely to have poor birth outcomes,” says lead author Dr. Tammy Chang, an assistant professor of family medicine at the U-M Medical School and member of the U-M Institute for Healthcare Policy and Innovation.

One quarter of all U.S. women become pregnant at least once by age 20. The U-M study focused on women ages 18-19, ages that are linked to the highest rates of unintended pregnancy.

Obese women who become pregnant have a higher risk for gestational diabetes, hypertensive disorders, blood clots, Cesarean-sections, stillbirths and birth-related injuries. Their infants are also more likely to be admitted to the neonatal intensive care unit. One fifth of U.S. adolescents are obese.

Authors note that obese adolescents have been shown to differ from normal-weight peers in several ways, including having lower self-esteem – which may hinder preparing for sex, asking clinicians for contraceptives or obtaining contraception from a pharmacy.