Preembryonic period: 1st week to end of 2nd week fertilization to formation of bilaminar germ disc Embryonic period: 3rd week to end of 8th week formation of embryo Fetal period: 9th week to birth growth Perinatal stage: 26th week to 4 week after birthPreembryonic period: 1st week to end of 2nd week fertilization to formation of bilaminar germ disc Embryonic period: 3rd week to end of 8th week formation of embryo Fetal period: 9th week to birth growth Perinatal stage: 26th week to 4 week after birth