Marijuana use among pregnant women is on the rise in the state of California, researchers have found.

According to a new study published in the December issue of the Journal of the American Medical Association, marijuana use during pregnancy has increased from 4.2 percent in 2009 to 7.1 percent in 2016.

There was a noted increase across all age ranges but the biggest jump observed was in younger women. In 2009, 12.5 percent of pregnant teens, aged 17 and younger, and 9.8 percent of pregnant women aged 18–24 admitted to using marijuana. By 2016, the figures have risen to 21.8 percent and 19 percent, respectively.

These findings are based on the medical records of 279,000 women in northern California who are on Kaiser Permanente's health care system. The participants, who went in for their standard prenatal care at eight weeks pregnant, were asked to complete a self-administered questionnaire on marijuana use since pregnancy. Their answers were corroborated with cannabis toxicology test.

The increasing trend for marijuana use among pregnant women has been noted for some time now. It is said to be the most commonly used illicit drug during pregnancy, specifically to combat ailments such as morning sickness or anxiety.

Previous studies, however, mainly relied on self-reports and thus, their statistics may not have been 100 percent reliable. This is because of underreporting due to possible social implications.

"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 healthcare system with 'gold standard' universal screening for prenatal marijuana use." authors Kelly Young-Wolff and Dr. Nancy Goler told Reuters.

In the new study, the researchers were able to determine that 55 percent who tested positive for marijuana use denied drug use on the questionnaire while 16 percent who admitted to using pot tested negative on the drug test.

One limitation of the study is that researchers were unable to determine whether positive drug tests for marijuana use were before or after the participants discovered that they were pregnant. People can still test positive even 30 days after using pot.

There is a growing concern over the increasing number of women using marijuana during pregnancy. Since it is still not clear how the psychoactive drug affects the fetuses in the womb as well as infants during breastfeeding, healthcare practitioners discourage the use of marijuana during this time.