Pregnancy

• Fertilisation

  The process where the sperm and egg unite to form a fertilised egg with a total of 46 chromosomes
• The mature human sperm has only 23 chromosomes
• The mature human egg has only 23 chromosomes
• The period from conception to birth is called pregnancy
• The normal duration of pregnancy is 265 days from conception to birth
• Pregnancy trimesters

  • First trimester
  • Second trimester
  • Third trimester
• Embryo

  The developing organism from conception until the end of the eighth week
• Fetus

  The developing organism after the eighth week
• The weeks refer to embryonic development and differ from clinical weeks taken from the last menstrual period
• Fertilisation takes place in the upper part of the fallopian tube
• Zygote

  The diploid cell resulting from the fusion of a sperm and an egg
• When the ovum is fertilised it becomes the diploid zygote
• Zygote to blastocyst

  1. Cleavage (cell divisions)

  2. 5 days after fertilisation

  3. 12-15 hours (2-cell stage)

  4. 24-30 hours (4-cell stage)

  5. 8-cell stage

  6. Morula

  7. Early blastocyst

  8. Late blastocyst
• Blastocyst

  A hollow ball of cells formed about 5 days after fertilisation
• The blastocyst consists of about 100 cells
• Inner cell mass


  Forms the embryo
• Embryonic tissues


  Can be cultured and collected to generate embryonic stem cells
• Hormones of pregnancy

  • HCG
  • Progesterone
  • Oestrogen
  • Prolactin
• HCG

  • Source: Chorion
  • Peak: 2nd month
• Progesterone

  • Source: Corpus luteum/placenta
  • Peak: 9th month
• Oestrogen

  • Source: Ovary/placenta
  • Peak: 9th month
• Prolactin

  • Source: Pituitary gland
  • Peak: After birth
• Types of twins

  • Identical twins
  • Fraternal twins
• Identical twins

  • 1 egg
  • 1 sperm
  • Same placenta
  • Separate amniotic sacs
• Fraternal twins

  • 2 eggs
  • 2 sperms
  • Separate placentas
  • Separate amniotic sacs
• The embryonic period

  1. Beginnings of adult organ systems

  2. Embryonic membranes developed

  3. Placenta developed
• Primary germ layers

  • Endoderm
  • Mesoderm
  • Ectoderm
• Endoderm

  • Pancreas
  • Liver
  • Bladder
  • Thymus
  • Thyroid gland
• Ectoderm

  • Skin
  • Hair
  • Nervous tissue
  • Lining of respiratory and digestive systems
• Mesoderm

  • Skeletal
  • Muscular
  • Cardiovascular
  • Lymphatic
  • Urinary
  • Reproductive systems
• Four embryonic membranes

  • Amnion
  • Yolk sac
  • Allantois
  • Chorion
• Week 1

  1. Fertilisation

  2. Formation of zygote

  3. Formation of morula

  4. Formation of blastocyst

  5. Blastocyst hatches

  6. Implantation begins
• Week 2

  1. Bilaminar germ disc forms

  2. Amniotic cavity & yolk sac appear

  3. Implantation completed

  4. Trophoblast starts to develop
• Week 3

  1. Trilaminar germ layers form

  2. Primitive groove & notochord appear

  3. Blood and vascular system develop
• Week 4


  1. Embryo distinguished as cylindrical structure

  2. Neural groove forms

  3. Somites visible

  4. Major organs begin to develop
• Week 5


  1. Recognisable embryo with head, face, tail and limb buds

  2. More than 30 somites

  3. Sense organs start to develop
• Week 6

  1. Umbilical cord starts to form

  2. Gut & liver develop

  3. Cartilaginous skeleton begins to develop
• Week 7

  1. Embryo recognisably human

  2. Head rounded

  3. Eyes conspicuous

  4. Hands & feet develop

  5. Trunk straightens

  6. Tail disappears

  7. Ossification of skeleton begins
• Week 8

  1. 27-31 mm long

  2. Face & limbs well developed

  3. First movements of extremities

  4. Over 90% of adult structures established

  5. Gender can be determined
• Foetal development occurs during the 9th week until birth