(Reuters Health) - Even as alcohol and tobacco use continue to decline among pregnant women in the U.S., a new study offers fresh evidence that more American mothers are using cannabis during pregnancy.

Other recent studies have also documented a rise in cannabis use among pregnant women of all ages, with some evidence of particularly sharp increases for teens and young adults. For the current study, researchers analyzed the proportion of pregnant women who used alcohol, tobacco or cannabis from 2002 to 2016.

Overall, the odds that pregnant women would use cannabis rose 3 percent a year during the study period, researchers report in JAMA Pediatrics. The increases were only seen in the first trimester and were most pronounced for women ages 26 to 44 and with at least a high school education.

Over the same period, the odds that pregnant women would use alcohol decreased two percent a year and the odds for cigarette use dropped three percent a year.

“Our findings are a reminder that while we are doing a decent job of discouraging pregnant women from using alcohol and tobacco during pregnancy, the message regarding potential adverse effects of cannabis exposure during pregnancy on fetal development is not getting out there,” said lead study author Arpana Agrawal of Washington University School of Medicine in Saint Louis.

The proportion of pregnant women using cannabis rose from less than 3 percent in 2002 to almost five percent by 2016, the study found.

During this period, the proportion of pregnant women using alcohol fell from about 10 percent to about 8 percent, and the proportion using cigarettes dropped from about 18 percent to about 10 percent.

The study can’t prove that cannabis, alcohol or tobacco impacts the health of pregnant women or their babies, nor can it explain why cannabis use rose while use of other substances declined.

“It is possible that the strong public health messages regarding the potential harms of alcohol and tobacco exposure during pregnancy have resulted in women being less likely to use them,” Agrawal said by email.

Doctors might also place more emphasis on warning women against the use of alcohol and cigarettes during pregnancy because these habits are more common than the use of cannabis or other drugs, Agrawal said.

“It is also possible that as cannabis use is increasingly viewed with permissiveness, or because it can be prescribed as a medicine in some states, there might be the misunderstanding that it is safe to use during pregnancy,” Agrawal added.

Some women may have heard that cannabis helps with pregnancy-related nausea, she noted. But most of the research linking cannabis to reduced nausea symptoms has involved cancer patients suffering from side effects of chemotherapy, and not pregnant women suffering from morning sickness.

The American College of Obstetricians and Gynecologists discourages doctors from prescribing or suggesting the use of marijuana for medicinal purposes while women are trying to conceive, pregnant, or nursing their babies.

SOURCE: bit.ly/2F2iXzQ JAMA Pediatrics, online November 5, 2018.