By Lindsey Tanner, AP Medical Writer

CHICAGO (AP) — U.S. women are increasingly using marijuana during pregnancy, sometimes to treat morning sickness, new reports suggest. Though the actual numbers are small, the trend raises concerns because of evidence linking the drug with low birth weights and other problems.

In 2014, almost 4 percent of pregnant women said they'd recently used marijuana, up from 2.4 percent in 2002, according to an analysis of annual drug use surveys.

Dr. Nora Volkow, director of the National Institute on Drug Abuse, said the results raise concerns and urged doctors and other health care providers to avoid recommending the drug for pregnant women. Volkow commented in an editorial published online Monday with the study in the Journal of the American Medical Association.

A separate study in the same journal found that almost 10 percent of adult marijuana users in the United States — 3 million people — have used it at least partly for medical reasons; 20 percent of these users live in states where medical marijuana isn't legal.

Volkow noted that laws legalizing medical marijuana in 29 states and Washington, D.C. do not list pregnancy-related conditions among allowed uses. But the laws also don't prohibit that use and don't include warnings about possible harms to the fetus, she said.

Strong evidence of harms is limited, but besides low birth weights, newborns whose mothers used marijuana while pregnant may face increased risks for anemia and other problems requiring intensive care. Memory and attention problems also been found in older children whose moms used marijuana in pregnancy, Volkow noted.

How marijuana might lead to those problems is unclear but Volkow said one theory is that it might interfere with formation of nerve cells and circuits in the brain during fetal development.

The American College of Obstetricians and Gynecologists discourages marijuana use by women who are pregnant or breastfeeding.

Both studies analyzed data from annual U.S. government surveys on drug use that are based on participants' self-reporting.

One focused on 200,510 women of reproductive age who participated in the 2002-2014 surveys. Recent use — within the past month — among non-pregnant women also increased over those years, from about 6 percent to 9 percent, researchers from Columbia University Medical Center reported.

Doctors "should screen and counsel pregnant women and women contemplating pregnancy about prenatal marijuana use," the researchers said.

The other study, led by the drug agency's Dr. Wilson Compton, focused on past-year marijuana use by nearly 100,000 adults aged 18 and up who participated in the 2013-14 drug survey.

About 13 percent said they had used marijuana; that translates to about 30 million adults. Overall, 90 percent used it for nonmedical reasons only and 6 percent used it only for medical reasons.

Prevalence of medical use was higher in states where that use is legal, but the researchers say the results suggest some doctors in other states may not feel bound by restrictions.

To Learn More:

The Risks of Marijuana Use During Pregnancy (by Nora D. Volkow, MD; Wilson M. Compton, MD, MPE; Eric M. Wargo, PhD, Journal of American Medical Association)

Use of Marijuana for Medical Purposes Among Adults in the United States (by Wilson M. Compton, MD, MPE; Beth Han, MD, PhD, MPH; Arthur Hughes, MS, et al; Journal of American Medical Association)

Surge in Drug-Affected Newborns Driven by Rising Rural-Area Female Opioid Use (by Lindsey Tanner, Associated Press)

Most Americans Support Medical Marijuana, But U.S. Government May be Stifling Needed Research (by Noel Brinkerhoff, AllGov)

Obama Administration Ends 16-Year Rule Forcing Non-Federally-Funded Marijuana Research to be Approved by Government (by Noel Brinkerhoff, AllGov)

Federal Government Finally Funds Research that Explores Positive Uses of Marijuana (by Steve Straehley, AllGov)