According to a new meta-analysis of studies, marijuana use during pregnancy is associated with negative effects on fetal development, such as low birth weight, increased likelihood of preterm birth, and even risk of newborn death.
The placenta is the most important organ in the metabolism between mother and fetus, delivering oxygen, nutrients and hormones for fetal growth. Damage to this organ increases the risk of complications for both parties.
Despite the potential harms associated with marijuana use before and after giving birth, the number of pregnant women using cannabis continues to rise. A 2019 analysis by the National Institute on Drug Abuse of more than 450,000 pregnant American women between the ages of 12 and 44 found that cannabis use more than doubled between 2002 and 2017 during that period. Most use occurred in the first three months of pregnancy and was mostly for recreational purposes, not medical reasons.
Alcohol use during pregnancy causes fetal alcohol syndrome
Smoking damages the developing lungs and brain of the fetus and causes sudden infant death syndrome (SIDS), according to the U.S. Centers for Disease Control and Prevention.
Taking opioids such as fentanyl, heroin and cocaine is widely known to cause birth defects, delayed fetal development and stillbirth, as well as a high risk of withdrawal syndrome in newborns.
These findings are confirmed even in the absence of clinical trials in which pregnant women were asked to use the substances in question and the results were compared with women who abstained. Such studies would be ethically impossible.
The quality of scientific evidence increases as the number of studies grows
A new study published in the journal JAMA Pediatrics analyzed 51 papers involving more than 21 million people.
The results showed that marijuana use during pregnancy increased the risk of preterm labor before 37 weeks by 52% and the risk of having a baby weighing less than 2,500 grams by 75%. Only six studies evaluated the effect of cannabis on mortality, and they found that the risk of infant mortality increased by 29% with use during pregnancy.
The GRADE method was used to assess the quality of evidence. In the previous analysis, conducted in 2024, the level of confidence was rated as very low or low, indicating that the findings were limited and unreliable. One year later, the confidence level improved to low to moderate, allowing more confidence in the use of these results, although future studies may refine the conclusions.
Historically, research on cannabis use has been complicated by its illegal status in most states and strict federal regulations. Current research shows that today's more potent marijuana is linked to cognitive impairment, risk of dementia, complications during surgeries, and increased risk of certain cancers. According to 2022 data, marijuana users require emergency care nearly 25 percent more often than non-users.
Even in the absence of pre-existing heart disease, any level of marijuana use increases the risk of stroke by 42% and heart attacks by 25%. Weed has been linked to arrhythmias, inflammation of the heart muscle, and other cardiovascular problems.
In young people, marijuana use has been linked to an increased risk of long-term mental health disorders, including depression, anxiety and schizophrenia, as well as social maladjustment and decreased academic success. Studies also show that daily use in adolescents and adults can lead to uncontrollable vomiting, and children born to marijuana-using mothers are more likely to have psychotic behavior, attention problems, sleep problems and cognitive impairment.
Serious maternal complications have been linked to marijuana use before and early in pregnancy
The study shows that pregnant women who used cannabis early in pregnancy had a 17% increased risk of gestational hypertension and an 8% increased risk of pre-eclampsia, according to lead author Kelly Young-Wolf of Kaiser Permanente's research department in Pleasanton, California. These women also had a 19% higher risk of placental detachment, which is life-threatening for mother and baby because the placenta provides oxygen and nutrients to the fetus.
Young-Wolf notes that pregnant women face a lot of misinformation about the safety of cannabis use from social media, vendors and peers. Our study adds to the growing body of evidence suggesting that cannabis use during pregnancy is unsafe.
It also found that women who used marijuana before or during early pregnancy were 9 percent more likely to be overweight and 5 percent more likely to be underweight, compared to the optimal weight for a successful pregnancy. Moore, an assistant professor at the Colorado School of Public Health, emphasized that «this is the largest study to date examining the effects of cannabis on maternal health» — and added that «it's never too late to limit or stop its use to avoid negative consequences».
Gestational hypertension is diagnosed when blood pressure rises above 140/90 after the 20th week of pregnancy, usually returning to normal after delivery. Persistently high blood pressure impedes blood flow and can lead to complications in the liver, kidneys, brain, uterus and placenta. In the case of pre-eclampsia, a more severe form of hypertension, there are signs of kidney or liver damage, decreased platelets, and pulmonary edema, which can cause seizures, stroke, and kidney failure.
According to Dr. Deborah Ensley, pre-eclampsia can lead to premature delivery and the need for hospitalization, which disrupts the establishment of skin-to-skin contact between mother and baby.
The study, published in the journal JAMA Internal Medicine, covered data on 250,000 pregnant women from Kaiser Permanente Northern California, where most underwent urine tests confirming marijuana use in the past 30 days. More than 20,000 women had used cannabis, of which nearly 11,000 did not admit to it but tested positive. Young-Wolf said that due to the large amount of data, the results showed that the risk of gestational hypertension increased with frequency of use, especially with daily use.
An association was also found between frequent use and an increased risk of placental detachment, as well as an increased chance of developing eclampsia, although statistical significance was not reached for this latter finding. The finding of a reduced risk of gestational diabetes was interesting, but Moore emphasized the need for more research to elucidate the effects of marijuana on metabolism during pregnancy.
The authors note limitations of the study: it was not possible to determine whether marijuana use continued throughout the pregnancy, and urinalysis only detected THC, with no data on route of administration. In addition, because all participants lived in the legalized state of California, the results cannot be generalized to the entire country, especially since the participants had higher incomes and private insurance.
Experts advise pregnant women to openly discuss cannabis use with their doctors and, if necessary, limit it or quit altogether to reduce health risks. Young-Wolf recommends that if a pregnant woman is not ready to stop using completely, she should do so less frequently and consult with a medical professional.