Customers shop for "Green Friday" deals at the Grass Station marijuana shop on Black Friday in Denver, Nov. 28, 2014. It was the first Black Friday since marijuana was legalized in Colorado Jan. 1, 2014. Reuters

As more states relax medical marijuana laws and legalize recreational use of the drug, the question of marijuana’s effect on fetal development has become part of the national debate on pot, but there’s little consensus among scientific researchers on pot smoking during pregnancy. The question of whether consuming marijuana during pregnancy could jeopardize the fetus’ health has been the focus of several scientific studies in recent decades yielding mixed results.

Lawmakers in Colorado rejected a bill Tuesday that would have made warning customers about the potential dangers of smoking pot while pregnant compulsory for marijuana dispensaries, the Associated Press reported. The decision came a day after the Colorado Department of Public Health & Environment released a comprehensive report on the health effects of marijuana use, including “mixed” evidence for pot’s influence on fetal development. The report concluded that while there was “no known safe amount of marijuana use during pregnancy,” there was “insufficient” evidence that consuming pot during pregnancy made offspring more likely to consume marijuana as minors, among other findings.

For one, it has been nearly impossible for researchers to get authorization to expose pregnant women and their fetuses to marijuana in clinical trials, according to Norml, a pro-marijuana group. Most research into the effects of marijuana on newborns has been conducted via surveys.

Some research has demonstrated that pot could affect a fetus’ brain development. One study published in January 2014 found that the main ingredient in marijuana, THC, could affect how brain cells were wired. "Our advice is that [pregnant] mothers should avoid marijuana," Tibor Harkany, a neuroscientist with the Karolinska Institute in Sweden, which led a study, told Live Science. Other studies have shown a potential link between THC and the formation of neurons as well as other cognitive effects, such as attention deficit disorder, in babies exposed to marijuana in utero.

But some research has demonstrated no direct relationship between a mother’s marijuana use and fetal health. A survey of 12,000 British women in 2002 found no significant differences in growth among babies exposed to marijuana in utero compared to those whose mothers did not consume pot. Similar results were found in a 1999 survey of nearly 13,000 Dutch mothers. Researchers concluded that marijuana use was “not a major prognostic factor regarding the outcome of pregnancy.”

Health experts have been divided on the issue. "We're just opening the door on research" into marijuana use during pregnancy, Dr. Brent Keeler told the AP.