The study, published in the Journal of the American Medical Association, used the medical records of 279,000 women residing in California who were on Kaiser Permanente, a health care service, Reuters reported.

Participating women were asked to answer a questionnaire and go through a drug test to see if they tested positive for marijuana use. Researchers found that marijuana consumption among pregnant women in the state has risen from 2009 to 2016 from 4.2 percent to 7.1 percent.

Kelly Young-Wolff, the study’s lead researcher and Dr. Nancy Goler said their research shows how marijuana might potentially harm an unborn baby.

“Our study is important because it addressed key limitations of prior studies by investigating trends in prenatal marijuana use using data from a large California health care system with gold standard universal screening for prenatal marijuana use,”

Goler and Young told Reuters.

Young-Wolff said marijuana usage increased the most in young females.

“We were concerned to find that the prevalence of marijuana use in pregnancy is increasing more quickly among younger females, aged 24 and younger, and to see the high prevalence of use in this age group,” Young-Wolff said.

The researchers noted marijuana was the “most commonly used illegal drug during pregnancy,” and could “impair fetal growth and neuro-development,” Reuters reported.

Barbara Yankey, a researcher at Georgia State University, told Reuters marijuana use may be on the rise because of the recent legalization of its recreational use “has made people think of the drug as less dangerous, even during pregnancy.”

“Because of the possibility of concurrent use of marijuana and other substances of abuse, the evidence of its direct association with preterm labor, fetal growth restriction, preterm birth, low birthweight and stillbirth is still debatable, though these adverse effects lean more towards an increased likelihood of occurrence,” Yankey continued.

The researcher said the amount of marijuana consumed and how often a person uses it is a factor.

Young and Goler did not condone the consumption of marijuana during pregnancy.

Officials in Colorado, where recreational pot was legalized, have warned women not to smoke it during pregnancy, Gizmodo reported. California will make recreational marijuana legal in 2018.