Cannabis a risk to fetal development, says UNLV School of Medicine study

working while pregnant
(Photo illustration by Daniel Berehulak/Getty Images)

Daily use of marijuana during pregnancy may lead to an increased risk of low birth weight, low resistance to infection, and can “disrupt the necessary flow of oxygen-rich blood through the placenta,” according to a study from the UNLV School of Medicine.  

The study is based on sonogram data from nearly 450 women who reported smoking marijuana daily during their pregnancies.  The women, who were deemed “high-risk” for a variety of reasons, were referred by their doctors for ultrasounds.

The study found “daily cannabis use is associated with delayed fetal growth, which can put a baby at risk of certain health problems during pregnancy, delivery, and even after birth. Those problems include low birth weight, hypoglycemia, low Apgar scores, among others, and in the most severe cases, delayed growth can lead to stillbirth.”

The study found no difference in fetal growth in 55 ultrasounds conducted during the first three months of pregnancy. But in the second trimester, ultrasounds detected “umbilical artery systolic to diastolic ratios were higher in marijuana users compared to nonusers.”   

Umbilical doppler results are a predictor of fetal growth.  

Third-trimester tests revealed 26 of 192 fetuses exposed to marijuana were growth-restricted compared with 6 of 192 fetuses of mothers who did not use.

“The physicians noted that the exposure to some chemical compounds that can be found in both tobacco and marijuana smoke may explain the growth abnormalities they observed,” said a release from UNLV.  “The compounds, known as polycyclic aromatic hydrocarbons, are present in both tobacco and marijuana smoke, but some studies have shown a higher concentration of these substances in secondhand marijuana smoke when compared to tobacco smoke, the physicians wrote.”

“Even after controlling for confounding variables, such as tobacco use, we still found the differences,” says Dr. Nora Doyle, Assistant Dean of Ultrasound Education and Professor of Maternal Fetal Medicine.

The report, published in The Journal of Maternal-Fetal and Neonatal Medicine, is at odds with earlier research from Washington University Medical School in St. Louis, which indicates “adverse outcomes appear attributable to concomitant tobacco use and other confounding factors.”  

That research used data compiled from 31 studies, and concluded “marijuana is not an independent risk factor for adverse neonatal outcomes after adjusting for confounding factors,” such as tobacco use. 

The UNLV study does not follow the outcomes of the pregnancies nor evaluate the health of the newborns.  

Medical professionals fear the spreading legalization of marijuana will fuel use among pregnant women.  

A study from the American College of Obstetrics and Gynecology says two to five percent of pregnant women self-report using marijuana while the prevalence increases to 15 to 28 percent among young, socioeconomically disadvantaged women.  

“Notably, 34 – 60 percent of marijuana users continue use during pregnancy, with many women believing that it is relatively safe to use during pregnancy and less expensive than tobacco,” the study says, adding 18.1 percent of pregnant women reporting marijuana use in the past year met criteria for “abuse, dependence, or both.”

Marijuana has long been used to quell nausea, even during pregnancy.  

Doyle said the researchers conducted no interviews with the mothers-to-be and had no information on their reasons for using cannabis during pregnancy.  

“They said marijuana made them feel better,” says Dr. Doyle. “It was just part of their daily life, inside and outside pregnancy.”

It’s unknown if any suffered from Hyperemesis Gravidarum, extreme nausea and vomiting that affects two to three percent of pregnant women. The condition landed Kate Middleton, Duchess of Cambridge, and comedy star Amy Schumer in the hospital during the early stages of their pregnancies.   

Experts say nausea is often the reason pregnant women turn to cannabis. 

“I smoked regularly but quit when I became pregnant,” says Nicole, who asked that we not use her last name. “My nausea was incapacitating.  I couldn’t work, eat or even go to the grocery store. I knew marijuana worked for cancer and AIDS patients with nausea so I tried it. It was immediate relief.” 

Nicole admits she worried about the effect of smoking marijuana on her developing baby, but feared her inability to eat and constant vomiting could be more perilous.   

A 2016 Kaiser Permanente study, funded by the National Institutes of Health, found women in Northern California who suffered severe nausea and vomiting during pregnancy were four times more likely to use marijuana than those who had no nausea. 

Women with mild nausea and vomiting had twice the odds of prenatal use than those with no nausea.  

“This is the largest study to date of nausea and vomiting in pregnancy and prenatal marijuana use,” lead author, Kelly Young-Wolff, PhD, a research scientist at Kaiser Permanente said in a news release. “Our findings add important evidence to a small but growing body of research suggesting that some pregnant women may use marijuana to self-medicate morning sickness.”

Of the 220,510 women screened in the first trimester, 5.3 percent in the Kaiser Permanante admitted to using marijuana or had the substance detected in urine.  Marijuana use by women reporting no nausea was 4.5 percent, while 2.3 percent of women with severe nausea used during pregnancy.

Dana Gentry
Senior Reporter | Dana Gentry is a native Las Vegan and award-winning investigative journalist. She is a graduate of Bishop Gorman High School and holds a Bachelor's degree in Communications from the University of Nevada, Las Vegas. Gentry began her career in broadcasting as an intern at Channel 8, KLAS-TV. She later became a reporter at Channel 8, working with Las Vegas TV news legends Bob Stoldal and the late Ned Day. Gentry left her reporting job in 1985 to focus on motherhood. She returned to TV news in 2001 to launch "Face to Face with Jon Ralston" and the weekly business programs In Business Las Vegas and Vegas Inc, which she co-anchored with Jeff Gillan. Dana has four adult children, a grandson, three dogs, three cats and a cockatoo named Casper.