Overview Time Landmark Day 0 Embryogenesis begins sperm fertilized ovum in ampulla all mitochondria are maternal in origin Week 1 Cleavage and blastocyst formation zygote undergoes mitotic divisions to form blastomeres → morula → blastocyst blastocyst cavity forms two cell masses inner cell mass = embryoblast which will become embryo outer cell mass = trophoblast which becomes placenta hCG secretion begins after implantation of blastocyst syncytiotrophoblast invades uterine wall and secretes hCG hCG detected in blood at day 8 and in urine at day 10 (pregnancy test) ↓ hCG: ectopic pregnancy or a sign for spontaneous abortion ↑ hCG: multiple pregnancy, hydatidiform mole, or gestational trophoblastic neoplasia Week 2 Bilaminar disk formation consists of 2 germ layers: epiblast and hypoblast 2 cavities are present: amniotic cavity and yolk sac placenta has 2 components: cytotrophoblast and synctiotrophoblast notochord forms Week 3 Gastrulation establishment of three embryonic germ layers (ectoderm, mesoderm, and endoderm) initiated by formation of primitive streak Formation of notochord and neural plate notochord induces its overlying ectoderm to become neuroectoderm. neuroectoderm will form neural plate, fully closes by week 4 neural plate forms neural tube and neural crest cells notochord will become adult nucleus pulposus in the vertebral column Week 3-8 Embryonic period major organs systems have begun development period in embryo is most susceptible to teratogens hematopoiesis begins around yolk sac at week 3 and is taken over by liver, thymus, spleen, and final and lifetime production is done in bone marrow week 7 embryo begins sexual differentiation Week 4 Initiation of heart beat Formation of upper and lower limb buds Closure of the neural plate Week 8 Fetal period fetus begins to move and has appearance of baby Week 10-12 Sex of fetus becomes recognizable Phenotypic differentiation completed by week 20