9) Foetal growth & development Flashcards
What is the foetal period?
Pre-embryonic period - Fertilisation > 3 weeks
Embryonic period - 3 - 8 weeks
Foetal period - 8 - 38 weeks
What is crown-rump length?
Measurement of the length of human embryos & foetuses
Top of the head (crown) to the bottom of the buttocks (rump)
When does crown rump length (CRL) increase most?
Rapidly in pre-embryonic, embryonic, and early foetal periods
Describe the main changes that take place as an embryo
Intense morphogenesis & differentiation
Little weight gain
Placental growth most significant
Describe the main change that take place in the early foetus
Protein deposition
Describe the main change that take place in the late foetus
Adipose deposition
Describe the changes in body proportion during the foetal period
Week 9 - head approx. 1/2 CRL
Then body length & LL growth accelerates
Birth - head approx. 1/4 of CRL
How is the respiratory system developed in the foetus?
Lungs develop relatively late, not needed until birth
Embryonic development creates bronchopulmonary tree only - airways, no gas-exchanging parts
Functional specialisation occurs in foetal period
Threshold of viability - only possible after 24 weeks
What is the pseudoglandular stage of development of the respiratory system?
Weeks 8-16
Duct systems begin to form within bronchopulmonary segments created during embryonic period
Bronchioles
What is the canalicular stage of development of the respiratory system?
Weeks 16-26 Formation of respiratory bronchioles Budding from bronchioles (formed weeks 8-16) May be viable at the end More vascular Some terminal sacs
What is the terminal sac stage of development of the respiratory system?
Week 26-term
Terminal sacs begin to bud from respiratory bronchioles
Some primitive alveoli
Differentiation of pneumocytes
- Type 1 - gas-exchange
- Type 2 - Surfactant production from week 20
What is the alveolar period of development of the respiratory system?
Late foetal-8 years
95% alveoli formed post-natally
How are the lungs prepared for immediate use after birth?
‘Breathing’ movement - conditioning of respiratory musculature
Fluid filled - Crucial for normal lung development
Which system begins development first and is last to finish?
Nervous system
Describe the development of the nervous system
Corticospinal tracts required for coordinated voluntary movements begin to form in the 4th month
Myelination of brain only begins in 9th month
Corticospinal myelination is incompete at birth. What is the evidence for this?
Increased infant mobility in the 1st year
When do movements of the foetus begin? (can be seen on USS)
Week 8
Large repertoire of movements develop
‘practising’ for post-natal life
e.g. suckling, breathing
The brain is the fastest developing organ in the foetus & infant. During the foetal period, what important changes occur?
Cerebral hemisphere becomes largest part of brain
Gyri & sulci form after 5 months, brain grows faster than head
Formation & myelination of nuclei & tracts
Relative growth of spinal cord & vertebral column
When do the sensory & motor systems develop?
Hearing & taste mature before vision
Organ of corti in inner ear well developed in foetus at 5 months
Retina is immature at birth
When does maternal awareness of foetal movements occur?
Week 17 onwards
Simple method of ante-partum surveillance
Reveals foetuses that require follow up
When is the definitive foetal heart rate achieved?
Around 15 weeks
Foetal bradycardia is associated with foetal demise
Describe the development of the kidneys
Ascent of kidneys complete at week 10
Foetal kidney function begins in week 10
(functional embryonic kidney is metanephros)
Renal pelvis, calyces week 23
Histological differentiation of cortex & medulla almost complete by 8 months
Foetal urine is a major contributor to what?
Amniotic fluid volume
Foetal kidney function is not necessary for survival during pregnancy, but what happens without it?
Oligohydramnios
Describe the development of the bladder
Lies in abdominal cavity in foetus & infant
Urine is emptied into amniotic fluid, to be swallowed by foetus
Bladder fills & empties every 40-60 minutes in foetus
What is the threshold of viability of a foetus?
Viability only possible once lungs have entered terminal sac stage of development
After 24 weeks
Viability only possible if brain is sufficiently mature to control body functions e.g. breathing
What is respiratory distress syndrome?
Often affects infants born prematurely
Insufficient surfactant production
If pre-term delivery is unavoidable or inevitable
- glucocorticoid treatment (of mother)
- increases surfactant production in foetus
What techniques can be used to assess foetal development?
USS
Doppler ultrasound
Non-Stress test (monitors heart rate changes associated with foetal movement)
Biophysical profiles (5 measured variables)
Foetal movements kick chart
What are the type of foetal growth restrictions?
‘growth restriction’ if weight below 10th percentile for gestational age
Symmetrical: Growth restriction is generalised & proportional
Asymmetrical: Abdominal growth lags, Relative sparing of head growth, Tends to occur with deprivation of nutritional & oxygen supply to foetus
Foetus’ born prematurely must be distinguished from those born at full term but are small. How can foetal age be estimated?
Duration of pregnancy - fertilisation age, age since last menstrual period
Developmental criteria - CRL, Foot length, Biparietal diameter of head, weight/appearance after delivery
Symphysis-Fundal height
What are the problems with measuring symphysis-fundal height?
Measured with tape measure:
- Number of foetuses can cause variation
- Volume of amniotic fluid can cause variation
- The lie of the foetus can cause variation
What daily rhythms does a foetus have?
Daily rhythms of HR, breathing & activity
What is oligohydramnios?
Too little amniotic fluid
Placental insufficiency
Foetal renal impairment
Pre-eclampsia
What is polyhydramnios?
Too much amniotic fluid Foetal abnormailty e.g inability to swallow, structural - blind-ended oesophagus, neurological - unable to coordinate swallowing movements
Which tests can be carried out to examine foetal body systems?
Foetal movement - Nervous, MSK
Foetal breathing movement - Respiratory, MSK, Nervous
Foetal tone - Nervous, MSK
Amniotic fluid volume - Urinary, GI, Utero-placental
Non-stress tests - CVS, Autonomic nervous
Describe the classification of birth weights
4.5kg = Macrosomia (maternal diabetes)
What may happen to the foetus is there is poor nutrition in early and late pregnancy?
Early pregnancy - Neural tube defects e.g. DiGeorge syndrome
Late pregnancy - Asymmetrical growth restriction, oligohydramnios
Describe the foetal circulation before birth
Oxygenated blood enters foetus via the umbilical vein from the placenta
Oxygenated blood bypasses the liver via the ductus venosus
Oxygenated blood passes from RA>LA via foramen ovale
Blood passes from pulmonary artery > aorta via ductus arteriosus
Deoxygenated blood returns to placenta via the 2 umbilical arteries
Why is resistance in the lungs extremely high before birth?
Hypoxic pulmonary vasoconstriction
(pulmonary arteries constrict in presence of hypoxia without hypercapnia, redirect blood flow to alveoli with higher O2 content)
Describe the foetal circulation after birth
First breath, removal of hypoxic pulmonary vasoconstriction, greatly reduced resistance of lungs
- Greater venous return to LA, LA pressure > RA, closure of foramen ovale (mins)
- Increased O2 sat of blood & decreased [prostaglandins], constriction of ductus arteriosus & umbilical artery (hrs)
- Stasis of blood in umbilical vein & ductus venosus, clotting of blood, closure due to fibrosis (days)
What is the function of amniotic fluid?
Surrounds foetus Mechanical protection (shock absorber) Moist environment so foetus doesn't dehydrate ~10ml at 8 weeks ~1l at 38 weeks Falls to ~300ml at 42 weeks
Amniotic fluid is turned over constantly, how is this achieved in early & late pregnancy?
Early - Amniotic fluid formed from maternal fluids
Foetal extracellular fluid by diffusion across non-keratinised skin
Later - Turnover via foetus
What does amniotic fluid contain?
Cells from foetus & amnion
Variety of proteins
Sampled by amniocentesis, diagnostically useful
What is the function of the foetal kidneys?
Metanephros - produces foetal urine
25 weeks ~100ml hypotonic urine a day
Term ~500ml urine a day
Adult! ~1 litre a day
The foetus swallows amniotic fluid constantly, why?
Absorbs water & electrolytes
Debris accumulates in foetal gut
Together with gut debris forms meconium
How is bilirubin eliminated from a foetus?
Bilirubin formed as result of Hb breakdown in foetus & mother Mother excretes bilirubin via bile Must be conjugated first Foetus cannot conjugate bilirubin Bilirubin accumulates, crosses placenta Excreted by mother
What happens if conjugation of bilirubin does not establish quickly in the neonate?
Neonate may become jaundiced
Liver never had to conjugate bilirubin before during pregancy
Takes time for liver to kick in
Exposure to light (phototherapy) stimulates liver to begin conjugation
Describe O2 transport in foetal blood
O2 diffuses across placenta (haemomonochorial)
Gradient of partial pressures
Placenta - large s.a. low resistance to diffusion
Maternal pO2 increased
Umbilical venous pO2 must be lower
O2 transport determined by umbilical artery pO2
Describe foetal O2 stores
Very low (~2mins worth)
Can be problematic in labour
Contraction of myometrium can compress placental blood vessels
Foetal HR good indicator of foetus O2 saturation
How is the O2 content of foetal blood increased?
Foetal pO2 is 4kPa
Normal arterial pO2 in adults is 13.3kPa
Foetal Hb different, without beta chains, better at lower pO2
Higher affinity of foetal Hb ‘sucks’ O2 across
Foetal Hb 70% saturated at 4kPa
Adult Hb 45% saturated at 4kPa
4kPa foetal blood then contains ~7.5mmol of O2 per litre, similar to normal adult
What is the double Bohr effect?
Increase in pCO2 of [H+] results in Hb losing affinity for & releasing more O2 (Bohr effect)
Happens in maternal & foetal blood (Double Bohr effect)
What is CO2 transfer between mother & foetus dependant on?
Partial pressure gradients
Foetus cannot tolerate higher pCO2 than mother due to acid base problems
Placental CO2 needs to be facilitated by lowering maternal pCO2
Achieved by hyperventilation, stimulated by progesterone
