Lecture 19- Foetal development Flashcards
The fetal period
- occurs after the embryonic period
- Growth and physiological maturation of structures created during the (v much shorter) embryonic period
- Period involving preparing for the transition to independent living after birth
pre-embyonic period
between weeks 1-2 (blastocyst)
embyronic period
WEEK 3- 9
fetal period
weeks 9-38
pregnancy weeks calculated form of
last menstrual period
i.e. conception weeks +2, so derm is 40 weeks
Pattern of growth during embryonic development
- Embryonic period is characterised by intense activity–>organogenetic period
- But absolute growth is very small
- Except placenta!
- Growth and weight gain accelerate in fetal period
what is used to assess gorwth
crown-rump lenght (CRL) and weight
- Crown-rump length (CRL)
increases rapidly in pre-embryonic, embryonic and early fetal periods
- Weight gain during development
- slow at first, then increases rapidly in mid- and late fetal periods
- Embryo- intense morphogenesis and differentiation, little wight gain, placental growth is most significant
- Early fetus- protein deposition
- Late fetus- adipose deposition
Antenatal assessment of fetal wellbeing
- Fetal movements
- Asking the mother if she has noticed changes in movement
- Regular measurements of uterine expansion
- Symphysis-fundal height
- Ultrasound scan
Obstetric ultrasound scan (USS)
- Safe
- Can be used early in pregnancy to calculate age
- Also to rule out ectopic, the number of foetuses’ etc
- Routinely carried out at 20 weeks
- Assess fetal growth
- Fetal anomalies
Estimation of fetal age based on 2 factors
-
LMP- last menstrual period
- Prone to inaccuracy
-
Developmental criteria
- Allow accurate estimation of fetal age
- Size e.g. CRL
- Growth curves
- Allow accurate estimation of fetal age
estimation of fetal age in trimester 1
Crown rump length
- Highly accurate when measuring between 7 and 13 weeks to date the pregnancy and estimate EDD
- Scan in T1 to check location, number and viability (heart beat)
estimation of fetal age in the 2 and 3 trimester
Biparietal diameter
- Distance between the parietal bones of the fetal skull
- Used in combination with other measurements to date pregnancy’s in T3 and T4
Abdominal cirucmferene and femur length
3- or 4-D USS
- New wave of obstetric ultrasonography
- Not currently replacing standard USS
Great at detecting potential congenital anomalies
classification of birth weights
- 3500g considered average
- <2500g suggests growth restriction
- >4500g is marcosomnia (maternal diabetes)
The importance of accurate dating
- Babies can have low birth-weight because
- They are premature
- Constitutionally small
- Small mother= small baby
- Suffered growth restriction (USS designed to pick this up)
- Neonatal morbidity and mortality
development of the respiratory system: the lungs develop relatively
late
- Not needed during embryonic or foetal life
in embryonic development which parts of the repsiratory system are created
bronchopulmonary tree
in the fetal period which parts of the respiratory system are developed
- Functional specialisation occurs in the fetal period
- Major implications for pre-term surivival
lungs are derived from
primitive gut tube (same as endoderm as GI tract)
respiratory system development: Week 8-16
pseudo glandular stage
- Duct system begins to form within the bronchopulmonary segments created during embryonic period
- Bronchioles
respiratory system development: week 16- 28
- Formation of respiratory bronchioles
- Budding from bronchioles formed during the pseudoglandular stage
respiratory system development: Weeks 26- term
- Terminal sac begins to bud from resp bronchioles
- Differentiation of Type I and II penuomocytes
The lungs during T2 and T3
- Gas exchange conducted at the placenta but lungs must be prepared to assume full burden at birth
- Breathing movements to practice during foetal development
- Conditioning of the respiratory musculature
- Breathing movement means amniotic fluid goes into the lungs à crucial for normal developmentà promotes differentiation
viability depends massively on the
how far the resp system is developed
- Viability is only possibility once the lungs have entered the
- terminal sac stage of development >24 weeks
- Can have abortion up till then
Respiratory distress syndrome
- Insufficient surfactant production by type 2 pneumocytes
- If pre-term is unavoidable or inevitable
- Glucocorticoid treatment (of the mother)
- Increases surfactant production in fetus
respiratory distress syndrome pathophysiology
The primary cause of RDS is inadequate pulmonary surfactant. The structurally immature and surfactant-deficient lung has ↓ compliance and a tendency to atelectasis; other factors in preterm infants that ↑ the risk of atelectasis are decreased alveolar radius and weak chest wall
The cardiovascular system
*
- Fetal cardiovascular system is arranged to ensure oxygenated blood collected by umbilical veins at the placenta and is circulated around the fetus
- Definitive fetal H/R is achieved at around 15 weeks
Fetal bradycardia associated with
fetal demise
fetal kidney function begins in
week 10
- Fetal urine major contributor of
- amniotic fluid volume
oligohydramnios
too little amniotic fluid due to:
- placental insufficiency–> not enough blood provided to be filtered by the kidney
- fetal renal impairment
polyhydramnios
too much amniotic fluid due to
- fetal abnormality such as inability to swallow
Fetal kidney function not necessary for survival in utero, but without it there is
oligohydramnios (lower than normal amniotic fluid)
the nervous system is the
- First to begin development and last to finish
nervous system development in the foetal period
- Corticospinal tracts required for coordinated voluntary movements begin to form in the 4th month
- Myelination of brain only begins in 9th month
- E.g. corticospinal tract myelination incomplete at birth, as evidenced by increasing infant mobility in the 1st year
Sensory and motor system
- No movement until 8th week
- At 8th week- startle movement
- Thereafter a large repertoire of movements develop
- Practising for post-natal life e.g. suckling (sucking thumb), breathing
- ‘quickening’
- Maternal awareness of fetal movement’s from 17 weeks onwards
- Felt earlier in subsequent pregnancy
- Low cost, simple method of antepartum fetal surveillance
- Reveal those foetuses requiring follow up
cerebellar devlopment and corticospinal tracts being to form at
week 16
myelination of spinal cord begins at
week 20
charactersitic gyri and sulci appear as cerebellar hemispheres frow larger than the skull
week 28
myelination begins in the brain
week 36
