A systematic review published Monday in JAMA Pediatrics concluded with moderate confidence on the basis of 51 studies with over 21.1 million participants that cannabis use during pregnancy can result in early births, low birth weights, and unusually small babies. The review also indicated significantly increased odds of miscarriages.

The study's authors, all but one of whom are based out of the Oregon Health and Science University in Portland, noted at the outset that the self-reported use of medical and nonmedical marijuana among pregnant women in the U.S. has more than doubled over the past two decades and continues to increase, despite warnings from the American College of Obstetricians and Gynecology.

According to the ACOG's Committee on Obstetric Practice, "self-reported prevalence of marijuana use during pregnancy ranges from 2% to 5% in most studies but increases to 15–28% among young, urban, socioeconomically disadvantaged women."

The OHSU researchers behind the new study noted further that marijuana is now reportedly the most commonly used federally illicit drug in pregnancy.

"This is cause for concern," wrote the researchers, because the main psychoactive component of cannabis, tetrahydrocannabinol, can cross the placenta and bind to endocannabinoid receptors on preborn babies' major organs.

The existing scientific literature already associates maternal marijuana use with numerous dire consequences, including stillbirth and fetal growth restriction as well as impaired cognition, decreased attention span, behavioral problems, and compromised visual-motor coordination in prenatally exposed children.

The researchers noted, however, that "clinicians are not consistently counseling patients regarding prenatal cannabis use, partly because of the limited and mixed available evidence."

'Ideally, it's best not to be exposed to THC.'

"Patients are coming to me in their prenatal visits saying, 'I quit smoking and drinking, but is it safe to still use cannabis?'" said lead author Jamie Lo, associate professor of obstetrics and gynecology in the OHSU School of Medicine. "Until direct harms have been proven, they perceive it to be safe to use."

"As the prevalence of prenatal cannabis use is rising, there is an urgent need for evidence-driven recommendations on the safety of use during pregnancy and while breastfeeding," Lo said in March.

Keen to furnish clinicians with a better understanding of the associated risks so that they can confidently counsel mothers regarding prenatal cannabis use, the researchers analyzed 51 observational studies including eight new studies, and raised the certainty of evidence for various potential adverse effects.

The researchers found that prenatal cannabis was linked to a 52% higher risk of premature birth and a 75% higher risk of low birth weight. They also found a 29% higher risk of infant death associated with the use of cannabis during pregnancy.

OHSU researchers revealed in another study recently published in the American Journal of Physiology-Lung Cellular and Molecular Physiology that a mother's use of cannabis while pregnant could adversely impact her baby's lung development and function, potentially resulting in asthma and other chronic respiratory health conditions.

In addition to jeopardizing the health of babies, marijuana has also been linked to various medical problems in adults, including cancer.

According to the Centers for Disease Control and Prevention, cannabis use can lead to increased risk of stroke, heart disease, and other vascular diseases; can harm lung tissues and cause scarring and damage to small blood vessels; and has been linked to depression, schizophrenia, and other mental illnesses.

Lo told CNN, "Ideally, it's best not to be exposed to THC, which is the psychoactive ingredient of cannabis, no matter what form you're using."

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