Embryology 3rd week to birth

• Functions of the embryo/fetus from 3rd month to birth

  • Protection
  • Nutrition
  • Respiration
  • Excretion
  • Hormone production
• Placenta


  Only mammals possess a placenta
• Human placenta


  • Discoid, haemochorial and deciduate
  • Attaches to uterine wall and establishes connection between fetus and mother

  
• Components of the placenta

  • Chorionic frondosum - Fetal Part
  • Decidua basalis - Maternal Part

  
• Implantation


  1. On D11 the blastocyst is surrounded on all sides by lacunar spaces around cords of syncytial cells

  2. Primary, secondary and tertiary villi successively develop from the stem villi

  3. An arterio-venous system develops in the mesenchymal core of each villus completed by D21

  
• Structural components of placental barrier or membrane

  • Maternal side - Basal plate
  • Fetal side - Chorionic plate
• Development of placenta


  1. Lacunar spaces become confluent and form a multilocular receptacle

  2. Decidua capsularis becomes thinner and regresses

  3. Exuberant growth of decidua basalis and chorion frondosum

  4. Discrete placenta formed from 6/52 and completed by 12/52

  5. Placenta grows in thickness and circumference until 16/52

  
• Placenta at term


  • Circular disc 15-20cm and 2.5 cm thick
  • Spongy, weighs about 500g
  • About 1:6 (proportionate weight of fetus at term)
  • Covers about 30% of uterine wall
  • Has 2 surfaces - fetal and maternal

  
• Types of placenta based on attachment of umbilical cord

  • Normal
  • Paracentral insertion
  • Marginal or Battledore
  • Velamentous

  
• Types of placenta based on shape

  • Discoid
  • Bidiscoidal
  • Oval
  • Triangular
  • Irregular
  • Lobed
  • Diffuse or placenta membranacea
  • Placenta Succenturiata
  • Fenestrated
  • Circumvallate

  
• Trimester


  Each last for 3 calendar months
• End of first trimester


  • All major systems develop
  • Crown-rump length (CRL) is used for measuring
• End of second trimester

  • May survive if born prematurely
• At 35 weeks


  • Fetus weighed 2500 gm, usually survives if born prematurely
• Measurements and characteristics of fetuses

  • Crown rump length (CRL), crown heel length (CHL)
  • Biparietal diameter (BPD)
  • Head circumference
  • Abdominal circumference
  • Femur length
  • Foot length
  • Fetal weight
• Crown rump length (CRL)


  Measurement from the top of the head, or crown, to the bottom of the buttocks, or rump
• Biparietal diameter (BPD)


  Maximum diameter of a transverse section of the fetal skull at the level of the parietal eminences
• Development from 9th to 12th week

  1. Head = 1/2 CRL at beginning of 9th week

  2. Legs are short, thighs are small at beginning of 9th week

  3. External genitalia show difference by end of 9th week

  4. Intestines return to abdomen by 11th week

  5. Fetal form of external genitalia established by 12th week

  6. Head < 1/2 CRL by end of 12th week

  7. Urine forms and discharges to amniotic fluid, which is swallowed by fetus between 9th to 12th week

  8. Primary ossification center appears
• Development from 13th to 16th week

  1. Rapid growth by 14th week

  2. Coordinated limb movement by 14th week

  3. Eye movements occur by 14th week

  4. Scalp hair pattern determined by 14th week

  5. Bones can be seen on radiographs by beginning of 16th week

  6. Ovaries differentiated by 16th week
• Development from 17th to 20th week

  1. Fetus grows 50 mm

  2. Fetal movements (quickening) felt by mother

  3. Skin is covered and protected by vernix caseosa

  4. Covered by lanugo (fine downy hair) by 20th week

  5. Eyebrows and head hairs are visible by 20th week

  6. Testes begin to descend by 20th week
• Development from 21st to 25th week


  1. Substantial weight gain

  2. Skin wrinkles and is translucent, pink to red

  3. Rapid eye movements begin

  4. Fingernails present

  5. Type II pneumocytes secrete surfactant by 24th week

  6. Born prematurely may survive under intensive care from 22nd to 25th weeks
• Development from 26th to 29th week


  1. Can breathe air by functioning lungs

  2. CNS controls rhythmic breathing and body temperature

  3. Eyes reopen

  4. Toenails are visible

  5. Subcutaneous fat flattens wrinkled skin
• Development from 30th to 34th week

  1. Pupillary light reflex by 30th week

  2. Born prematurely usually survive by 32nd week

  3. Born as normal weight (premature by date) by 32nd week

  4. White fat = 8% body weight by 32nd week
• Development from 35th to 38th week

  1. Nervous system is mature for integrative functions

  2. Circumference of head is equal to that of abdomen

  3. CRL 360 mm, weight 3400 gm

  4. Slow growing before birth

  5. Full term - Bluish-pink skin, Testes in scrotum