Cannabinoids are chemicals like THC, the primary psychoactive compound in marijuana, that act on cannabinoid receptors in neurons, repressing the normal release of neurotransmitters.
"We know from limited human studies that use of marijuana in early pregnancy is associated with risks, including miscarriage, birth defects, developmental delays and learning disabilities, but animal research suggests that the potential for many more developmental issues are linked with the drug," said G. Ian Gallicano, Associate Professor at Georgetown University in the US.
Cannabinoids affect many aspects of human development because THC and other chemicals alter molecular pathways that should not be disrupted during development of a foetus, the researchers said.
THC is also known as a promising agent for treating cancer, because it negatively affects tumour growth and can cause the death of cancer cells.
However, embryo development has similarities to tumour formation -- it turns against growth pathways that are necessary for development.
"The fact that THC seems to stop cancer growth suggests how damaging the chemical could be for a foetus," Gallicano added.
For the study, the researchers reviewed scientific literature on cannabinoids and embryonic development published between 1975 and 2015.
The findings in human cells studies showed that THC has a half-life of eight days in fat deposits and can be detected in blood for up to 30 days.
The THC also readily crosses the human placenta, which can slow clearance of the drugs while prolonging foetal exposure, and increasing the risk of damaging it.
In addition, THC and other cannabinoids have been found to interfere with the use of folic acid (vitamin B9), which has long been known to be essential for normal development and growth of the human placenta and embryo.
Deficiencies in folic acid are also linked to low human birth weight, increased risk of spontaneous abortion, and neural tube defects such as spina bifida, the researchers observed.
The study was published in the journal BioMed Central (BMC) Pharmacology and Toxicology.
