THURSDAY, May 9, 2024 (HealthDay News) -- Smoking cigarettes while pregnant has long been known to harm the fetus, but new research shows things get even worse when marijuana is in the mix.

The study by a team at Oregon Health & Science University (OSHU) in Portland involved more than 3 million pregnancies.  

It found heightened risks for underweight newborns, preterm delivery and even infant death among women who used tobacco and cannabis while pregnant.

“With the growing legalization of cannabis around the country, there is often a perception that cannabis is safe in pregnancy,” study co-author Dr. Jamie Lo said in an OSHU news release. 

“Because we know that many people who use cannabis often use tobacco or nicotine products, we wanted to better understand the potential health implications on both the pregnant individual and the infant," she explained. Lo is associate professor of obstetrics and gynecology (maternal-fetal medicine) at OHSU.

The findings were published May 7 in JAMA Network Open.

Lo's team analyzed hospital discharge data and vital statistics on more than 3 million pregnant women in California averaging about 29 years of age.  

Of those women, just over 23,000 (0.7%) said they had used cannabis while pregnant; close to 57,000 (1.8%) had smoked tobacco; and more than 10,?300 (0.3%) had used both substances while pregnant.

Compared to women who abstained from cigarettes or marijuana during pregnancy, the odds for infant death doubled among women who smoked either tobacco or cannabis, the researchers found.

But the risk was even higher among women who used both tobacco and cannabis while pregnant, rising to four times that of abstainers. 

Similar trends were seen for other pregnancy outcomes, such as having an underweight newborn or delivering prematurely. Women who used cannabis and tobacco while pregnant had double the risk of those outcomes compared to women who used neither drug.

The researchers understand that, for some women, quitting both substances at once can be especially tough.  

"We acknowledge the complexities of individual circumstances may make this goal challenging, and for some patients is simply not realistic," said study lead author Dr. Adam Crosland, an assistant professor of obstetrics and gynecology at OHSU.

Still, "our findings suggest that avoiding use of just one of these substances can decrease the pregnancy risks we see when both substances are used together, which is a critical piece of information providers can highlight when counseling patients," he said.

More information

Find out more about the dangers of smoking during pregnancy at the March of Dimes.

SOURCE: Oregon Health & Science University, news release, May 7, 2024