1. The zygote phase — Period-I (weeks 1 – 2 after fertilization) consists of cell division and implantation of this cell mass in the uterus. During this phase, teratogen would cause loss of the conceptus | 1. The zygote phase — Period-I (weeks 1 – 2 after fertilization) consists of cell division and implantation of this cell mass in the uterus. During this phase, teratogen would cause loss of the conceptus |