Stress causes decreased cell accumulation in early periimplantation embryos and the placental trophoblast stem cells derived from them. Benzopyrene and many other stressors activate stress enzymes that lead to suppressed stem cell accumulation through diminished proliferation and increased apoptosis. Trophoblast stem cells proliferate and a subpopulation of early postimplantation trophoblast cells differentiate to produce the first placental hormones that arise in the implanting conceptus. These hormones mediate antiluteolytic effects that enable the continuation of a successful implantation. The normal determination and differentiation of placental trophoblast stem cells is dependent upon a series of transcription factors. But, these transcription factors can also be modulated by stress through the activity of stress enzymes. This review enumerates and analyzes recent reports on the effects of benzopyrene on placental function in terms of the emerging paradigm that placental differentiation from stem cells can be regulated when insufficient production of stem cells is caused by stress. In addition, we review the other effects caused by benzopyrene throughout placental development.