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Updated: Mar 6 2023

Embryonic Period Timeline

  • 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
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