1. embryonic development (fertilisation → week 8)
   1. week 1 (day 0-7)
      • fertilisation (day 0)
      • pre-implantation events
      • implantation: adherence (day 6)
      • implantation: invasion (day 7)
        • syncytiotrophoblasts secrete hCG (day 7)
   2. week 2 (day 7-14)
      • primary stem villi (day 11-13) (placenta from beginning of week 2)
      • implantation: completion (day 12)
   3. week 3 (day 14-21)
      • primitive streak (day 13-14)
      • gastrulation (day 14-16)
      • secondary stem villi (day 16)
      • start of organogenesis (day 17) (halfway through week 3)
      • tertiary stem villi (day 21)
      • embryo most vulnerable to Thalidomide (week 3-8)
   4. week 4 - 8 (day 21-56)
      • gonadal ridge (week 5)
      • sexual differentiation (week 6-8)
      • Pax6 first expressed (week 7)
      • ossification begins (week 8)
2. fetal development (week 9 → birth)
   • hCG peaks (week 9)
   • placenta matured (week 12/ 14 LMNP)
   • hCG baselines (week 18)
   • surfactant production in lungs (week 18-20)
   • testicular descent
     • transabdominal phase (3 months to birth)
     • inguinoscrotal phase (6 months to birth)
3. postnatal development