Session 9 - Foetus and Foetal Growth Flashcards
Define the pre-embryonic period
Fetilisation -> 3 weeks
Define the embryonic period
3 -> 8 weeks
Define the foetal period
8-38 weeks
What is the main method of measuring foetal size?
Crown Rump Length
What growth happens in the embryo?
Intense morphogenesis and differentiation
Little weight gain
Placental growth most significant
What growth occurs in early foetus?
Protein deposition
What growth occurs in late foetus
Adipose deposition
What parts of respiratory system develop in the embryonic phase?
Bronchopulmonary tree, with airways but no gas exchanging parts
What significance does the late development of respiratory system have?
o Major implications for pre-term survival
Threshold of Viability
Viability is only a possibility after 24 weeks
What are the four stages of resp system development
Pseudoglandular stage
Canalicular
Terminal sac stage
Alveolar period
What is pseudoglandular stage?
o Weeks 8 – 16
o Duct systems begin to form within the bronchopulmonary segments created during the embryonic period
Bronchioles
What is canalicular stage?
o 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
What is terminal sac stage?
o Week 26 – Term
o Terminal sacs begin to bud from the respiratory bronchioles
o Some primitive alveoli
o Differentiation of pneumocytes
Type 1 – Gas exchange
Type 2 – Surfactant production from week 20
What is alveolar period?
o Late fetal 8 years
95% of Alveoli are formed post-natally
How do the lungs prepare for birth?
Breathing’ movement
Conditioning of the respiratory musculature
o Fluid filled
Crucial for normal lung development
What is main part of brain development?
Corticospinal tract
When does corticospinal tract begin to develop?
4th month
Why can’t babies move properly at birth?
o Myelination of the brain only beings in the 9th month
Corticospinal tract myelination incomplete at birth, as evidence by increased infant mobility in the 1st year
When does movement begin?
Week 8
How much of body weight is brain at birth?
12%
Give four changes which occur in brain during foetal period
o Cerebral hemisphere becomes the largest part of the brain
Gyri and sulci form after 5 months as the brain grows faster than the head
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
In what order do senses develop
Hearing and taste mature before vision. The organ of corti in the inner ear is well developed in the fetus at 5 months, but the retina is immature at birth.
What is quickening?
o Fetal movements can be seen by USS at Week 8
o Maternal awareness of fetal movements from Week 17 onwards
What is the use of assessing foetal movements
o Low cost, simple method of ante-partum fetal surveillance
o Reveals fetuses that require follow-up
What is the significance of foetal heart rate?
Bradycardia associated with foetal demise
When is ascent of kidneys complete
Week 10
Outline kidney development
o Fetal kidney function begins in week 10
Functional embryonic kidney is the Metanephros
o Renal pelvis, calyces etc present by week 23
o Histological differentiation of cortex and medulla almost complete by 8 months
o Fetal urine is a major contributor to amniotic fluid volume
o Fetal kidney function is not necessary for survival during pregnancy, but without it there is oligohydramnios.
How often do bladder fill and empty?
o Bladder fills and empties every 40 – 60 minutes in the fetus (seen on USS)
What is the threshold of viability?
Viability is only a possibility once the lungs have entered the terminal sac stage of development (after 24 weeks).
What is respiratory distress syndrome
o Often affects infants born prematurely
o Insufficient surfactant production
o If pre-term delivery is unavoidable or inevitable
Glucocorticoid treatment (of the mother)
Increases surfactant production in the fetus
Give six techniques to assess foetal development
o Ultrasound Scan o Doppler ultrasound o Non-Stress Tests (NST) Monitors hear-rate changes associated with fetal movement o Biophysical profiles (BPP) 5 measured variables o Fetal movements kick chart
When is a foetus regarded as having growth restriction?
If its weight is below the 10th percentile for gestational age
Give two types of growth restriction
Symmetrical growth restriction
Asymmetrical growth restriction
What is symmetrical growth restriction?
Growth restriction is generalised and proportional
Asymmetrical growth restriction
Abdominal growth lags
Relative sparing of head growth
Tends to occur with deprivation of nutritional and oxygen supply to fetus
Why is estimation of foetal age important?
It is important to distinguish between a fetus born prematurely and one born full term but small.
Give two methods of dating a pregnancy?
o Fertilisation age
o Age since mother’s Last Menstrual Period (LMP)
Give problems with
o Fertilisation age
o Age since mother’s Last Menstrual Period (LMP)
o Fertilisation age
Use of calendar months may cause inaccuracies
o Age since mother’s Last Menstrual Period (LMP)
Irregular cycles may cause confusion
Give five developmental criteria of foetus
o Crown-Rump (CR) length Used in T1 o Foot length o Biparietal diameter of head Used in T2/T3 o Weight after delivery o Appearance after delivery
What is another way of measuring foetal age?
Symphysis – Fundal height
o Distance between symphysis pubis to top of uterus (fundus)
o Measured with a tape measure
Give three problems with Symphysis – Fundal height
Number of fetuses can cause variation
Volume of amniotic fluid can cause variation
The lie of the fetus can cause variation
What is use of daily rhythmns?
A fetus has daily rhythms of heart rate, breathing and activity. Heart rate variability is a good index of developing control systems.
What is oligohydraminos?
o Too little
o Placental insufficiency
o Fetal renal impairment
o Pre-eclampsia
PWhat is polyhramnois?
o Too much o Fetal abnormality E.g. inability to swallow Structural – blind-ended oesophagus Neurological – unable to coordinate swallowing movements
What is quickening?
o Maternal awareness of fetal movements from Week 17 onwards
o Low cost, simple method of ante-partum fetal surveillance
o Reveals fetuses that require follow-up
Classify birth weights
o < 2,500g = Growth Restriction
o 3,500g = Average
o > 4,500g = Macrosomia
o Maternal diabetes
Describe the effects of poor nutrition in early pregnancy
o Neural tube defects
E.g. DiGeorge Syndrome
Describe the effects of poor nutrition in late pregnancy
o Asymmetrical Growth Restriction
Subsequent oligohydramnios
Describe foetal 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 so high in the lungs?
Hypoxic Pulmonary Vasoconstriction.
Describe changes in foetal circulation after birth
The infant takes its first breath, removal Hypoxic Pulmonary Vasoconstriction and greatly reducing the resistance of the lungs. 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)
Give two main roles of aminiotic fluid
Mechanical protection
Moist environment so foetus does not dehydrates
How much amniotic fluid at
8 weeks
38 weeks
42 weeks
o ~10ml at 8 weeks
o ~1 Litre at 38 weeks
o Falls to ~300ml at 42 weeks
How is amniotic fluid produced early in pregnancy?
Formed from maternal fluids
Fetal extracellular fluid by diffusion across non-keratinised skin
How is amniotic fluid produced later in preg?
Turnover via foetus
What do amniotic fluid contain? What is it useful?
Cells from foetus and amnion.
It included a variety of proteins, and if sampled via a Amniocentesis, can be diagnostically useful.
How much urine does foetus produce at 25 weeks
and
term
25 weeks - 100ml
term - 500ml
Why does foetus swallow amniotic fluid? What does it form?
Absorbs water and electrolytes
Debris accumulates in foetal gut and forms meconium
What is bilirubin formed by? Who conjugates it?
Haemoglobin breakdowwn in foetus and mother
Conjugated by mother
What problems can occur with bilirubin in foetus? What can cure it?
o Neonate may become jaundiced if conjugation does not establish quickly
Liver has never had to conjugate bilirubin before during pregnancy, so it takes a little bit of time for the liver to kick in
Exposure to light (phototherapy) stimulates the liver to begin conjugation
What is the driving force for oxygen diffusion from mother to foetus?
Oxygen diffuses across the placenta from maternal blood across a thin barrier
The driving factor for this is 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 must be must lower
What is dangerous about foetal hypoxia?
Fetal oxygen stores are very low (about 2 minutes worth), which can be a problem in labour, particular if the problem involves the placenta. Contraction of the myometrium can compress placenta blood vessels.
What is a good indicator for foetal O2?
Foetal heart rate is a good indicator of foetus O2 saturation.
What is normal foetal pO2?
o Fetal pO2 is 4kPa
Normal arterial pO2 in adults is 13.3kPa
What is different about foetal haemoglobin?
o Fetus has different haemoglobin, without beta chains, which is better at these 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 is double bohr effect?
An 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.
What is CO2 transfer moderated by?
Placental CO2 therefore needs to be facilitated by lowering maternal pCO2. This is achieved by Hyperventilation, stimulated by Progesterone.
