Describe the pattern of weight gain through embryonic and fetal periods and relate this to aims of the different stages
Weight gain is slow at first, but increases rapidly in the mid and late fetal periods.
Embryo
▪ Intense morphogenesis and differentiation
▪ Little weight gain
▪ Placental growth most significant
Early fetus
▪ Protein deposition
Late fetus
▪ Adipose deposition
What is Crown Rump Length (CRL) and when does it increase the most?
Crown Rump Length (CRL) is the measurement from top of fetus to bottom - increases rapidly in the pre-embryonic, embryonic and early fetal periods.
Body proportions change dramatically during the fetal period. How does week 9 compare with that at birth?
o At week 9, the head is approximately half of the crown rump length
o At birth the health is approximately one quarter of the crown rump length
Which part of the respiratory system develops in the embryonic period?
o Embryonic development creates only the bronchopumonary tree
▪ Airways, no gas exchanging parts
(The lungs develop relatively late, as they are not needed until birth)
When does functional specialism of the lungs occur? Outline the stages
o Functional specialisation occurs in the fetal period
Pseudoglandular Stage: Weeks 8 – 16
o Duct systems begin to form within the bronchopulmonary segments created during the embryonic period
Canalicular Stage: Weeks 16 – 26 o Formation of respiratory bronchioles ▪ Budding from bronchioles formed during the pseudoglandular stage o May be viable at the end o More vascular o Some terminal sacs
Terminal Sac Stage: Week 26 – Term
o Terminal sacs begin to bud from the respiratory bronchioles
o Some primitive alveoli
o Differentiation of pneumocytes
Alveolar Period
o Late fetal - 8 years
What are the two types of pneumocytes?
▪ Type 1 – Gas exchange
▪ Type 2 – Surfactant production from week 20
During T2 and T3 gas exchange occurs at the placenta. How do the lungs prepare for full function immediately after birth?
o ‘Breathing’ movement
▪ Conditioning of the respiratory musculature
o Fluid filled
▪ Crucial for normal lung development
What are the implications of the relatively late development of the respiratory system?
o Major implications for pre-term survival
▪ Threshold of Viability
▪ Viability is only a possibility after 24 weeks
What tracts are required for coordinated voluntary movements and when do they begin to form?
Corticospinal tracts - 4th month?
What system is the first to start developing and the last to finish?
Nervous system
Describe the neurological changes that occur in the fetus
o Cerebral hemisphere becomes the largest part of the brain
o Histological differentiation of cortex in the cerebrum and cerebellum
o Formation and myelination of nuclei and tracts
o Relative growth of the spinal cord and vertebral column
The fetal cardiovascular system is arranged to ensure oxygenated blood collected by the umbilical vein at the placenta is circulated around the fetus.
When is the definitive fetal heart rate achieved?
15 weeks - Fetal bradycardia is associated with fetal demise
What can happen if fetal kidneys do not function properly?
Fetal kidney function is not necessary for survival during pregnancy, but without it there is oligohydramnios
Fetal kidney function begins in week 10, what is the name of the functional embryonic kidney?
Metanephros
Where does urine come from/ where does it go?
o Urine is emptied into the amniotic fluid, to be swallowed by the fetus.
o Bladder fills and empties every 40 – 60 minutes in the fetus (seen on USS)
What three factors influence the viability of the preterm neonate?
1) Threshold of Viability - Viability is only a possibility once the lungs have entered the terminal sac stage of development (after 24 weeks).
2) Brain Development - Viability is only possible if the brain is sufficiently mature to control body functions, e.g. breathing.
3) Respiratory Distress Syndrome
o Often affects infants born prematurely
o Insufficient surfactant production
If pre-term delivery is unavoidable or inevitable, how is mother and/or fetus treated?
Glucocorticoid treatment (of the mother) - Increases surfactant production in the fetus
List techniques used to assess fetal development
o Ultrasound Scan o Doppler ultrasound o Non-Stress Tests (NST) o Biophysical profiles (BPP) o Fetal movements kick chart
What is the difference between symmetrical and asymmetrical growth restriction?
o Symmetrical Growth Restriction
▪ Growth restriction is generalised and proportional
o Asymmetrical Growth Restriction
▪ Abdominal growth lags
▪ Relative sparing of head growth
▪ Tends to occur with deprivation of nutritional and oxygen supply to fetus
What are the limitations of using fertilisation age or age since mother’s last period to predict fetal age?
o Fertilisation age - Use of calendar months may cause inaccuracies
o Age since mother’s Last Menstrual Period (LMP) - Irregular cycles may cause confusion
What is the significance of daily rhythms?
A fetus has daily rhythms of heart rate, breathing and activity. Heart rate variability is a good index of developing control systems.
With regard to amniotic fluid, what is oligohydramnios and what is it associated with?
o Too little
o Placental insufficiency
o Fetal renal impairment
o Pre-eclampsia
What is the name for too much amniotic fluid and what fetal abnormalities is it associated with?
o Polyhydramnios o Fetal abnormality ▪ E.g. inability to swallow ▪ Structural – blind-ended oesophagus ▪ Neurological – unable to coordinate swallowing movements
Outline the three categories of birth weight. What is the heaviest associated with?
< 2,500g = Growth Restriction
3,500g = Average
> 4,500g = Macrosomia - Maternal diabetes
Describe how poor maternal nutrition effects early and late pregnancy. Give examples of associated conditions.
Poor Nutrition in Early Pregnancy
o Neural tube defects
▪ E.g. DiGeorge Syndrome
Poor Nutrition in Late Pregnancy
o Asymmetrical Growth Restriction
▪ Subsequent oligohydramnios
Describe Fetal circulation before birth
o Oxygenated blood enters fetus via the Umbilical Vein from the placenta
o Oxygenated blood bypasses the liver via the Ductus Venosus
o Oxygenated blood passes from the RA –> LA via the Foramen Ovale
o Blood passes from the pulmonary artery –> Aorta via the Ductus Arteriosus
o Deoxygenated blood returns to the placenta via the two Umbilical Arteries
Why is resistance in fetal lungs extremely high?
Hypoxic Pulmonary Vasoconstriction
After birth, the infant takes its first breath, removing what?
Hypoxic Pulmonary Vasoconstriction - greatly reducing the resistance
Describe the changes in circulation one the infant takes its first breath and reduces the resistance?
o Greater venous return to LA
▪ Pressure in LA > RA
▪ Closure of the Foramen Ovale
▪ (Minutes)
o Increased O2 saturation of blood and decreased [Prostaglandins] (placenta has been removed)
▪ Constriction of Ductus Arteriosus
▪ Constriction of Umbilical Artery
▪ (Hours)
o Stasis of blood in Umbilical Vein and Ductus Venosus
▪ Clotting of blood
▪ Closure due to subsequent fibrosis
▪ (Days)
What is the function of amniotic fluid? Outline changes in quantities
Amniotic fluid surrounds the fetus, providing mechanical protection (shock absorber) and a moist environment so the fetus does not dehydrates
o ~10ml at 8 weeks
o ~1 Litre at 38 weeks
o Falls to ~300ml at 42 weeks
What does amniotic fluid contain? How is this clinically useful?
Amniotic fluid contains cells from the fetus and amnion. It includes a variety of proteins, and if sampled via a Amniocentesis, can be diagnostically useful.
What is the name of the functional embryonic kidney and what does it produce?
Metanephros, which produces Fetal Urine.
o At 25 weeks the fetus produces ~100ml of hypotonic urine a day
o At term the fetus produces ~500ml of urine a day (Adults only produce ~1 Litre a day!)
The fetus swallows amniotic fluid constantly, absorbing water and electrolytes - what happens to the debris?
▪ Debris accumulates in the fetal gut
▪ Together with gut debris forms Meconium.
What happens to fetal bilirubin? Why may some neonates be jaundice?
o Fetus cannot conjugate bilirubin
▪ Bilirubin crosses placenta (after accumulating)
▪ Excreted by mother
o Mother excretes bilirubin via bile (Must be conjugated first)
o Neonate may become jaundiced if conjugation does not establish quickly
What is the driving factor for oxygen diffusing across the placenta from maternal blood across a thin barrier (Haemomonochorial).
The gradient of partial pressures between maternal and umbilical blood, as the placenta has a large area for and low resistance to diffusion.
o Maternal pO2 increased
o Umbilical venous pO2 lower
Oxygen transport rate is determined by what?
The umbilical artery p02,
What measure is a good indicator of foetus O2 saturation?
Foetal heart rate
How does fetal haemoglobin differ from adult haemoglobin?
o Fetus haemoglobin is without beta chains and is better at lower partial pressures of O2.
▪ The higher affinity of fetal haemoglobin ‘sucks across’ the O2
▪ Fetal Haemoglobin is 70% saturated at 4kPa
▪ Adult Haemoglobin is 45% saturated at 4kPa
What do we mean by the ‘double Bohr effect’?
Increase in pCO2 or [H+] concentrations results in Haemoglobin losing affinity for and releasing more oxygen. This is called the Bohr Effect. This happens both in the maternal and fetal blood, so is termed the Double Bohr Effect
Transfer of CO2 is dependent on partial pressure gradients. How is maternal CO2 kept lower than fetal?
This is achieved by Hyperventilation, stimulated by Progesterone