Researchers and clinicians have long warned women not to use marijuana while they are pregnant or nursing. They agree that infants' exposure to tetrahydrocannabinol, or THC, demonstrably changes their brain development. But their studies are limited. Legally, child protective services would have to step in if a child tests positive for the drug—a challenge for researchers who want to figure out how much THC infants absorb and what this means for them in the long term.

Meanwhile, marijuana laws are loosening, and attitudes about cannabis have shifted. Physicians who witness the trends up close fear there will be another public health crisis that will hurt children.

"We are in the opioid crisis due to expanding prescriptions for opioids with little thought to the consequences of widespread use, including use during pregnancy," said Dr. Lauren M. Jansson, director of pediatrics for the Johns Hopkins Center for Addiction and Pregnancy. "My fear is that we will see the same thing with marijuana."

The forthcoming study from Colorado researchers is the first to show how tenaciously THC can live in breast milk. The authors, neonatologist Dr. Erica Wymore and pediatrician Dr. Maya Bunik, work out of Children's Hospital Colorado. The state is ground zero for legalizing recreational use of the drug in the U.S.

But the researchers' deepening concerns don't seem to be hitting the broader medical community amid changing public views, the growth of the cannabis industry and a backlash against the former stigma around marijuana use. Jansson has applied for grants to study marijuana-dependent mothers and impacts of cannabis on their infants before and after birth but, she said, those applications haven't been well-received.

"There's no interest there, right now," she said. "So I'm doing preliminary research and describing the effects I'm seeing."

Bunik and Wymore had to overcome financial hurdles too, even though the Centers for Disease Control and Prevention originally commissioned their study. The agency ended up halting the funds after budget cuts, so the hospital and Colorado's health department had to pitch in money so they could complete their work.

This is especially frustrating for them because of what they have already found.

Jansson, whose expertise is in opioids and maternal opioid use disorder, has worked on maternal addiction issues clinically since 1991 and as a researcher since 2000. In children exposed only to cannabis, she is seeing behavior dysfunctions she said she can't ignore. Earlier this summer, she co-authored an article for JAMA highlighting preliminary research that traced prenatal exposure's adverse effects on "infant neurobehavior and child development up through the teen years."

Exposure after birth could exacerbate these effects, her study said. She noted impaired memory, aggression, sleep disturbances and other "developmental and behavioral concerns."

She emphasized she is seeing these effects in kids who have been exposed to marijuana alone, not combined with alcohol, nicotine or other drugs.

Bunik and Wymore have also noted issues with "cognition, executive function and early depression" in children ages 8 to 12 whose mothers self-reported exposure.

"We know that self-report actually underestimates" use, Bunik added.

Meanwhile, states continue to relax laws around cannabis, and public perception toward marijuana use is changing too. Nine states and the District of Columbia allow both medical and recreational use and 21 additional states allow medical use.

Wymore and Bunik told Modern Healthcare that women increasingly are using marijuana to curb pregnancy symptoms like nausea and mood swings, and then they are finding it hard to quit once the baby arrives. Out of the 30 to 50 women who signed up for the study, only 10 were able to abstain from marijuana for the entire course.

"The moms said, 'I couldn't deal and needed to smoke,' " Bunik said. "That for us is concerning. You probably need to start discussions [about use] early in pregnancy. By the time the baby arrives they're accustomed to using as part of a coping mechanism."

About 70% of Colorado's cannabis dispensaries recommended marijuana products to pregnant women to treat their first-trimester nausea, according to an August study by the American College of Obstetricians and Gynecologists.

"Few dispensaries encouraged discussion with a healthcare provider without prompting," the study concluded. "As cannabis legalization expands, policy and education efforts should involve dispensaries."

The American College of Obstetricians and Gynecologists, or ACOG, has also found that 34% to 60% of marijuana users keep using during pregnancy "with many women believing that it is relatively safe to use during pregnancy and less expensive than tobacco." A 2015 analysis in the American Journal of Obstetrics and Gynecology found that more than 70% of pregnant women see no or slight risk to using marijuana once or twice a week.

In September, the American Academy of Pediatrics published a finding that 8.5% of young pregnant women between 18 and 25 reported past-month marijuana use in 2016.

And like the studies on post-birth exposure to cannabis, research on fetal impact is also limited. The ACOG has concluded that frequent use appears to increase the risk of low birth weight, while less-than-weekly marijuana use does not. Its clinical guidance has also made note of several studies that showed "statistically significantly smaller birth lengths and head circumferences as well as lower birth weights" among infants whose mothers used marijuana during their pregnancies.

The findings were "more pronounced among women who used more marijuana, particularly during the first and second trimesters."

The government and the healthcare industry do not know what these trends currently cost or will cost in the future. Researchers have had a difficult time quantifying medical expenses from maternal marijuana use because the effects on exposed infants are similar to those in infants whose mothers used tobacco, alcohol or other drugs during pregnancy. Past research found that women who smoked tobacco or drank while they were pregnant were likelier to use cannabis as well.

A 2017 white paper from Truven Health Analytics estimated that premature infants or infants with a low birth weight typically cost Medicaid an average of nearly $20,000, or $14,000 above the average cost of a baby born at a healthy weight or without other complications. All told, the analysis found, low birth-weight babies or otherwise complicated births cost Medicaid an additional $3 billion each year. The report said states should focus on reducing premature births in order to save money.

Colorado does not analyze the claims data that would pinpoint costs. Nearly 14% of the nearly 30,000 Colorado babies born in 2015 whose deliveries were paid for by Medicaid were low birth weight. Of the total babies born that year, 27% went to the neonatal intensive-care unit, and 2% were preterm. In 2016, the numbers shifted slightly: about 23% of babies went to the NICU, nearly 7% were low birth weight, and 3% were preterm.

But the state's Medicaid department, which provided these numbers, did not have a breakdown for the causes of these complications, and a department spokesperson noted that claims data would not be a reliable way to track them. She added that this kind of research would require "significant resources."

The Colorado Legislature this year introduced nearly 30 marijuana-related bills, but so far no measure that would address usage around pregnancy, according to staff for the state's Speaker of the House. Colorado became the first U.S. state to legalize recreational cannabis use in late 2012 and has been ground zero for the marijuana industry's explosive growth and subsequent regulation.

The state's health department has published strong guidance for providers with adamant warnings.

"If patient desires a pregnancy, discuss importance of cessation of marijuana and other potentially harmful substances," the guidance stated. "Consider use of contraception while the patient is working towards cessation of substances."