(Reuters Health) - One in five American women don’t start using birth control when they first start having sex, increasing their risk of unwanted pregnancies, a U.S. study suggests.

Women who don’t use condoms, birth control pills or any other forms of contraception when they first become sexually active are almost four times more likely to have an unwanted pregnancy in the next three months than their counterparts who use birth control from the start, researchers report in Pediatrics.

“More and more women are using some method to prevent pregnancy in their first month of sexual activity, and most of this increase can be attributed to condom use,” said lead study author Dr. Mara Horwitz of the Harvard Pilgrim Health Care Institute in Boston.

“The fact that condoms account for the majority of timely contraceptive initiation suggests that teenagers and young adults in the U.S. are reaching for condoms over other contraceptive methods when they first become sexually active,” Horwitz said by email.

“Early awareness of and access to contraception may help reduce teen unwanted pregnancy,” Horwitz added.

American women today typically become sexually active around age 17, and unintended pregnancy rates are highest among women ages 18 to 24, researchers report in Pediatrics.

Lack of contraception the first time women have vaginal sex has long been associated with an increased risk of unintended pregnancies, the study authors note. Previous research also suggests that contraceptive choices women make during their first sexual encounter may be linked with their odds of using effective contraception, or their risk of having an unwanted pregnancy or abortion, up to a decade later.

Data for the current study came from a nationally representative sample of 26,359 women who had sex for the first time between 1970 and 2014. During that time, the proportion of women who began using birth control before they started having sex rose from less than 10 percent to more than 25 percent.

Over the same period, 493 women, or almost 2 percent, reported an unwanted pregnancy within three months of becoming sexually active. Delayed contraception initiation was associated with almost four times the odds of an unwanted pregnancy.

Among women who said they did start contraception in a timely fashion, 229 still had an unwanted pregnancy within three months. The type of birth control wasn’t associated with the odds of an unintended pregnancy.

Non-white women and poor women were more likely to delay contraception initiation than white or more affluent women.

This suggests that that accessibility and affordability may play a role, said Devon Hensel, an adolescent health researcher at Indiana University School of Medicine in Indianapolis who wasn’t involved in the study.

“Being a young woman of color and/or being poor makes it less likely that adolescents will have access to the most important factors impacting their contraceptive use - some of these include being able to afford long-acting effective contraceptive methods, having regular contact with a doctor to evaluate difficulties with and/or needed changes to contraceptive method, having concerns about confidentiality with their health care provider and experiencing discrimination within the health care system,” Hensel said by email.

The study wasn’t a controlled experiment designed to prove whether or how the timing of birth control initiation or the type of contraception used might directly impact the risk of teen pregnancy or unplanned pregnancy.

Even so, the results underscore the importance for young adults of having a parent or doctor they can speak freely to about contraception, said Dr. Kate Lucey, a pediatrics researcher at Northwestern University in Chicago who wasn’t involved in the study.

“Ideally, teenagers should have a contraception plan in place before they start having sex, and parents play an important role in this plan,” Lucey said by email. “Parents who don’t believe in, or don’t feel comfortable, discussing contraception options with their teen should view their teen’s doctor as a source of unbiased, confidential information about contraception for their teen.”

SOURCE: bit.ly/2SRwdKg Pediatrics, online January 15, 2019.