(CBS) Adding to concerns about the harms of teenage pregnancy are new U.S. survey results that show 14 percent of teenage mothers-to-be smoke marijuana.

This large national survey found that more than twice as many pregnant 12- to 17-year-olds use marijuana as their nonpregnant peers. And significantly more use the drug than pregnant women in their 20s, said Dr. Nora Volkow, director of the U.S. National Institute on Drug Abuse.

Evidence regarding pot’s effects on the developing fetus is limited, but the American College of Obstetricians and Gynecologists (ACOG) recommends that pregnant women stop using the drug.

“Some of the studies that do exist suggest that there are risks to the pregnancy from pot use,” said Dr. Judy Chang, associate professor of obstetrics, gynecology and reproductive sciences at the University of Pittsburgh.

Those risks include “scrawnier babies, kids who have some problems with their thinking and learning abilities, [and] kids who find it harder to do more complicated brain tasks when they are teenagers,” said Chang, who wasn’t involved in the survey.

Teen pregnancies are already associated with smaller babies partly because young moms-to-be are less likely to receive prenatal care than older women. They’re also more likely to smoke during pregnancy, says ACOG.

The new findings stem from a 2002-2015 National Survey on Drug Use and Health that explored marijuana use among 410,000 females ages 12 to 44. About 14,400 survey participants were pregnant at the time of polling.

All participants were asked about their pregnancy status and pot use during the prior month. Final tallies were based on self-reports, which could mean the results underrepresent true usage.

“We found that almost 4 percent of [all] pregnant women reported marijuana use in the past month,” said Volkow.

Pot use was much lower for adults 26 and older (less than 2 percent) than for those 18 to 25 years old or teens 12 to 17.

Also, blacks were more likely than whites or Hispanics to use pot during pregnancy, the survey found.

Volkow said her team was “surprised” at the difference between pregnant and nonpregnant teens who used pot -- 6 percent of nonpregnant teenagers, compared to slightly more than 14 percent who were expecting.

“Though we cannot establish causality, it is very likely that teens who are more prone to risk-taking generally may be more likely to both use marijuana and to have unprotected sex,” said Volkow.

The survey also revealed that pot use in pregnancy was more common during the first trimester, “when fetuses may be most susceptible to damage from drugs,” the investigators said.

That “could indicate that some women may be using marijuana for managing their nausea, and/or that they were unaware that they were pregnant,” Volkow said.

Chang wasn’t surprised by the findings.

“Studies are showing increased acceptance and use of marijuana among adolescents and increased use among pregnant women,” she noted. “So it’s not surprising to see increased use among pregnant adolescents.”

But Chang’s own research and other studies found that “even when pregnant patients say that they are using pot to treat morning sickness, the use actually started before they got pregnant,” she added.

No matter the underlying reason, Chang suggested that concerns over the long-term impact deserve more scientific attention.

“We know that pot affects the brain. That is what causes its effects,” she said. “What those other studies suggest is that there could be a risk of causing brain damage in a developing baby with pot smoking during pregnancy.”

Volkow agreed. The component of marijuana that causes the high -- the cannabinoid THC (tetrahydrocannabinol) --- may also influence neural development and brain maturation, she said.

Prior research suggests that in-womb exposure to even low amounts of THC may increase the newborn’s long-term risk for addiction, Volkow said.

The survey results appear in the April 17 issue of the Annals of Internal Medicine.