Pregnancy

• Pregnancy


  The period from conception to birth, when a woman carries a developing fetus in her uterus
• Normal duration of human pregnancy

  265 days from conception to birth
• Pregnancy calculation


  280 days (40 weeks) from the first day of the last menstrual period
• Trimesters


  The pregnancy is broken down into three 3-month periods
• Embryo


  The developing organism from the time of conception until the end of the eighth week, after which it is known as a fetus
• Fertilisation


  Sperm fertilises ovum to form zygote

  
• Zygote


  The diploid cell resulting from the fusion of a sperm and an egg

  
• Cleavage


  Cell divisions
• Blastocyst


  A hollow ball of cells formed about 5 days after fertilisation, consisting of an outer cell layer that will form the placenta and an inner cell mass that will form the embryo
• Embryonic membranes


  • Amnion
  • Yolk sac
  • Allantois
  • Chorion
• Primary germ layers


  • Endoderm (inner)
  • Mesoderm (middle)
  • Ectoderm (outer)

  
• Structures derived from primary germ layers

  • Ectoderm: Skin, hair, nervous tissue, lining of respiratory & digestive systems
  • Mesoderm: Skeletal, muscular, cardiovascular, lymphatic, urinary & reproductive systems
  • Endoderm: Pancreas, liver, bladder, thymus & thyroid gland

  
• Placenta


  Develops from the trophoblast, facilitates exchange of nutrients, waste and gases between mother and fetus
• Stages of embryonic development

  • Weeks 1-3: Pre-embryonic stage
  • Weeks 4-8: Embryonic stage
  • Weeks 9-birth: Fetal stage
• Key events in embryonic development

  • Week 1: Fertilisation, formation of zygote, morula, blastocyst
  • Week 2: Bilaminar germ disc forms, amniotic cavity & yolk sac appear, implantation completed, trophoblast starts to develop
  • Week 3: Trilaminar germ disc forms, primitive groove & notochord appear, blood & vascular system start to develop
  • Week 4: Embryo 1.5-3.5 mm, neural groove forms, 4-12 somites visible, many major organs begin to develop
  • Week 5: Recognisable as embryo with head, face, tail & limb buds, 2.5-7 mm long, >30 somites, sense organs start to develop
  • Week 6: Umbilical cord starts to form, gut & liver start to develop, cartilaginous skeleton begins
  • Week 7-8: Embryo recognisably human, head rounded, eyes conspicuous, hands & feet develop, trunk straightens, tail disappears, ossification of skeleton begins

  
• Key events in fetal development

  • Weeks 9-12: Foetus doubles in length, fingernails, toes and hair develop
  • Weeks 12-16: Foetus grows rapidly to 18 cm long and 100 g, fingerprints appear, movements detected
  • Weeks 16-20: Foetus 25 cm long and 330 g, movements felt by mother
  • Weeks 20-24: Foetus 27-35 cm long and 565-680 g
  • Weeks 24-28: Foetus over 1000 g and 38 cm long, moves vigorously
  • Weeks 28-32: Foetus 41-45 cm long and 1800-2200 g
  • Weeks 32-36: Foetus ~50 cm long and 2500-4500 g, reduced activity
  • Week 40: Foetus at term, head smaller in proportion to body