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Haibin Wang, PhD, Research Assistant Professor at Vanderbilt University Medical Center, and Huirong Xie, PhD, a Postdoctoral Fellow in the Department Of Cell And Developmental Biology at Vanderbilt University Medical Center, et al., wrote in their 2006 study of mice, "Fatty Acid Amide Hydrolase Deficiency Limits Early Pregnancy Events," published in the Journal of Clinical Investigation:

Exposure to Delta-9-THC [one of the active ingredients in marijuana] causes retarded development and oviductal retention of embryos…

We speculated that these developmentally retarded embryos eventually fail to implant in the uterus… When examined on day 5, most of the mice (7 or 8) exposed to THC failed to show any sign of implantation…

These findings reinforce the idea that during normal pregnancy, locally produced endocannabinoids elicit an ‘endocannabinoid tone’ conductive to synchronous development and timely journey of embryos through the oviduct for on-time implantation, whereas an exposure to exogenous cannabinoid [that which enters from outside the body] ligands such as THC swamps this endocannabinoid tone, leading to early pregnancy failure.

2006