Four states in the U.S. allow for recreational use of marijuana while 24 allow the use of medical marijuana. However, using marijuana during pregnancy can have adverse effects on the developing fetus. Unfortunately, according to researchers, there is “far too little research” available on the effects of cannabinoids on embryo development.

A study published Thursday in the journal BioMed Central (BMC) Pharmacology and Toxicology urged the need for more papers analyzing the effects of marijuana, smoked or eaten by mothers, on the health of newborns.

Researchers from Georgetown University Medical Center combed through papers studying cannabinoids and their effect on human embryos, using mostly animal models published between 1975 and 2015. They found that Tetrahydrocannabinol or THC, the active ingredient in marijuana, can cross the placenta thus exposing the fetus to the chemical.

“We know from limited human studies that use of marijuana in early pregnancy is associated with many of the same risks as tobacco, including miscarriage, birth defects, developmental delays and learning disabilities, but animal research suggests the potential for many more developmental issues linked with the drug,” G. Ian Gallicano, the study’s senior investigator, said in a statement.

“We also know that THC is a promising agent for treating cancer, because it negatively affects tumor growth and can cause the death of cancer cells. Embryo development has similarities to tumor formation — it turns on growth pathways that are necessary for development,” he added. “The fact that THC seems to stop cancer growth suggests how damaging the chemical could be for a fetus.”

Human cells studies revealed that THC has a half-life of eight days in fat deposits and can be detected in the blood for up to 30 days. A study of pregnant dogs found that maternal tissues can act as reservoirs for THC, researchers said.

Cannabinoids can affect the use of folic acid, which is important for normal growth and development of the placenta and the embryo. A deficiency in folic acid can cause low birth weight, increase the risk of spontaneous abortion and neural tube defects like spina bifida.

Researchers also found that THC levels in marijuana that is smoked has increased 25-fold since 1970. The studies, however, did not analyze the harmful effects of smoking marijuana in the animals.

“All of the model systems point to the notion that cannabinoids affects many aspects of human development because THC and other chemicals alter molecular pathways that shouldn’t be disrupted during development of a fetus,” Gallicano said. Disruption of BDNF, an important neural pathway, can increase the risk of congenital disorder and can impair cognition as seen in autism.