Fetal Growth and Development Flashcards
What is the fetal period?
Growth and physiological maturation of the structures created during the (very much shorter) embryonic period.
Period involving preparation for the transition to independent life after birth
Weeks 9 to 38
In terms of growth and activity how do the embryonic and fetal period compare?
Embryonic period is characterised by intense activity
- Organogenetic period
- But absolute growth is very small
- Except placenta
Growth & weight gain accelerate in fetal period.
Describe the differential growth of a fetus (from pre-embryonic).
CRL (crown-rump length) increases rapidly in the pre-embryonic, embryonic & early fetal periods
Weight gain is slow at first, then increases rapidly in mid- and late fetal periods
- Embryo - intense morphogenesis & differentiation; little
weight gain; placental growth most significant
- Early fetus - protein deposition
- Late fetus - adipose deposition
How do body proportions change during fetal period?
Dramatically
- At 9 week, the head is approx. half CRL
- Thereafter, body length & lower limb growth
accelerates
What are the ante-natal assessments of fetal well-being?
Mother - Fetal movements Regular measurements of uterine expansion - Symphysis-fundal height Ultrasound scan
Tell me about the obstetric ultrasound scan (USS)
Safe
Can be used early in pregnancy to calculate age
- Also: rule out ectopic, number of foetuses etc.
Routinely carried out at ~20 weeks
- Assess fetal growth
- Fetal anomalies
How do you estimate fetal age?
From last menstrual period (LMP) - Prone to inaccuracy Developmental criteria - Allow accurate estimation of fetal age - Crown-rump length (CRL) - Biparietal diameter - Abdominal circumference & femur length
Tell me about crown-rump length.
Measured between 7 & 13 weeks to date the pregnancy and estimate EDD (estimated date of delivery)
Scan in T1 also used to check location, number, viability
Tell me about biparietal diameter.
The distance between the parietal bones of the fetal skull
Used in combination with other measurements to date pregnancies in T2 & T3
Tell me about abdominal circumference & femur length.
AC & FL used in combination with BPD for dating and growth monitoring
Also useful for anomaly detection
Tell me about 3-D or 4-D USS.
New wave of obstetric ultrasonography
Not likely to replace standard USS
- Complimentary tool
What are the different classifications of birth-weights?
3500g is considered average
4500g is macrosomia
- Maternal diabetes
Many factors influence birth weight, not all pathological
What is the importance of accurate dating?
Babies can have low birth-weight because
- They are premature OR
- They are constitutionally small OR
- They have suffered growth restriction
- Associated with neonatal morbidity & morality
Describe the development of the respiratory system in the fetus.
The lungs develop relatively late
- Embryonic development creates only the
bronchopulmonary tree
- Functional specialisation occurs in the fetal period
- Major implication for pre-term survival
Weeks 8-16: pseudoglandular stage
- Duct system begins to form within the
bronchopulmonary segments created during the
embryonic period
- Bronchioles
Weeks 16-26: canalicular stage
- Formation of respiratory bronchioles
- Budding from bronchioles formed during the
pseudoglandular stage
Weeks 26 - term: terminal sac stage
- Terminal sacs begin to bud from the respiratory
bronchioles
- Differentiation of:
- Type I & Type II pneumocytes
- Surfactant
Describe the lungs of the fetus during T2 & T3 of pregnancy.
Gas exchange conducted at placenta, but lungs must be prepared to assume full burden at birth
“Breathing” movements
- Conditioning of the respiratory musculature
Fluid filled
- Crucial for normal lung development
How are the lungs related to implications for pre-term survival?
Threshold of viability
- “Cannot be continually pushed back since there is a
limit beyond which the lungs will not be sufficiently
developed to sustain life”
- BMA briefing document on fetal viability
Viability is only a possibility once the lungs have entered the terminal sac stage of development
- i.e. >24 weeks
Explain respiratory distress syndrome.
Often affects infants born pre-maturely
Insufficient surfactant production
If pre-term delivery is unavoidable or inevitable
- Glucocorticoid treatment (of the mother)
- Increases surfactant production in fetus
Tell me a little bit about the development of the fetal cardiovascular system.
The fetal cardiovascular system is arranged to ensure oxygenated blood collected by umbilical vein at the placenta is circulated around the fetus
The definitive fetal H/R is achieved at around 15 weeks
- Fetal bradycardia is associated with fetal demise
Tell me a little bit about the development of the fetal urinary system.
Fetal kidney function begins in week 10
Fetal urine is a major contributor to amniotic fluid volume
Fetal kidney function is not necessary for survival in utero, but without it there is oligohydramnios
What is the importance of amniotic fluid volume?
Oligohydramnios
- Too little
- Placental insufficiency, fetal renal impairment
Polyhydramnios
- Too much
- Fetal abnormality - e.g. inability to swallow
Tell me a little bit about the development of the fetal nervous system.
First to begin development and last to finish
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 evidence by increasing infant mobility in the 1st year
Tell me a little about the movement of fetus’.
No movement until the 8th week
Thereafter a large repertoire of movements develop
- “Practising” for post-natal life
- e.g. suckling, breathing
What are the implications of fetal movements?
“Quickening”
- When there is maternal awareness of fetal movements
from 17 weeks onwards
- Low cost, simple method of ante-partum fetal
surveillance
- Reveal those fetuses requiring follow-up
