Prenatal Development By claudi99 Jan 1, 2017 start of the period of the zygote Jan 14, 2017 end of the period of the zygote Mar 5, 2017 start of the period of the Embryo Mar 18, 2017 end of the period of the embryo May 20, 2017 start of the period of the fetus Jul 22, 2017 end of the period of the fetus Aug 1, 2017 start of the first trimester