After a nursing woman smokes marijuana once, her baby through her breast milk will consume traces of the drug's chief psychoactive element for at least six weeks and possibly longer, according to a soon-to-be-released study out of Colorado. 

For physicians who see cannabis-associated birth complications and long-term brain development concerns with children, the research is another step to try to square growing public nonchalance about marijuana with medical guidelines about use.

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. 

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