Lecture 17: Fetal development Flashcards
What happens during the fetal period?
Growth and physiological maturation of the structures created in the embryonic period
What are the different stages from fertilisation to birth?
- pre-embryonic (1-2 weeks)
- embryonic (3-8 weeks, where risks of congenital structural defects is highest)
- fetal (9-38 weeks, brain is only structure susceptible to damage here as it is still developing)
When are pregnancy weeks counted from?
First day of the last menstrual period
What is the pattern of growth in the embryonic period?
-intense activity
-organogenetic period
-absolute growth is very small (except placental growth)
(we see growth and weight gain accelerate in the fetal period)
What is a good measurement of size in the early stages of pregnancy?
Crown rump length (CRL)
-increases rapidly in the pre-embryonic, embryonic and early fetal periods
What causes weight gain in the fetus?
Embryo: intense morphogenesis and differentiation, little weight gain, placental growth most significant
Early fetus: protein deposition
Late fetus: adipose deposition (particularly brown adipose)
What is the CRL at 38 weeks?
36 cm
How does the fetus’ body proportions change throughout the pregnancy?
9 weeks: head is half the CRL
Fetal period: body length and lower limb growth accelerates
(even at birth the head is a quarter of the CRL)
How do we do an ante-natal assessment of fetal wellbeing?
- fetal movements (seen in 2nd trimester)
- regular measurements of uterine expansion (symphysis-fundal height)
- USS (safe)
What is the purpose of an obstetric USS?
-can be used in early pregnancy to calculate age
-rule out ectopics
-number of fetuses
(routinely carried out at 20 weeks: assess fetal growth and see fetal abnormalities as structures are big enough)
How do we estimate fetal age?
- LMP: prone to inaccuracy
- developmental criteria e.g. size/growth curves (allows accurate estimation of fetal age)
What is the crown rump length?
From top of head to bottom of torso
-measured b/w 7-13 weeks to date the pregnancy and estimate the EDD
(good as little biological variance at this stage)
What is the biparietal diameter?
Distance between the parietal bones of the fetal skull
What is used for dating and monitoring growth after 13 weeks?
-biparietal diameter
-abdominal circumference
-femur length
(usually done at 20 week scan)
What is 4D USS?
New wave of obstetric ultrasonography
- not currently replacing standard USS, used as a complimentary tool
- really good at detecting congenital abnormailities
How do you classify birth weights?
3500g is average
<2500g suggests growth restriction
>4500g is macrosomia (maternal diabetes)
(many factors influence birth weight, not all pathological)
Why can babies have a low birth weight?
- premature (low birth weight is not abnormal here)
- constitutionally small (small mother)
- suffered growth restriction (associated with neonatal morbidity and mortality): ante-natal screening used to pick up on this group
What are the last structures to develop?
Lungs/respiratory system
- embryonic development creates only the bronchopulmonary tree
- functional specialisation occurs in the fetal period
Where do the lungs develop from in the embryonic period?
- respiratory diverticulum is the outpouching of the primitive gut tube (endoderm)
- tracheoesophageal septum separates the respiratory and GI tracts
- respiratory diverticulum grows into the trachea and bronchi branch off (only make airways during the embryonic period)
What is the stages of lung development in the fetal period?
Pseudoglandular stage: duct system begins to form within the bronchopulmonary segments created during the embryonic period, forming bronchioles
Canalicular stage: formation of respiratory bronchioles
Terminal sac stage: terminal sacs begin to bud from the respiratory bronchioles, differentiation of type 1 and 2 pneumocytes to develop the alveolar membrane- allow for surfactant production
How are the lungs prepared to assume full burden of gas exchange at birth?
Gas exchange is conducted at the placenta but lungs need to be ready at birth
- ‘breathing’ movements: allow for conditioning of the respiratory musculature
- lungs are fluid filled with amniotic fluid: crucial for normal lung development
What is the threshold of viability?
Fetal viability cannot be continually pushed back as there is a limit beyond whichthe lungs will not be sufficiently developed to sustain life
-viability is only a possibilty once the lungs have entered the terminal sac stage of development e.g. 24 weeks
What is respiratory distress syndrome?
-often affects infants born prematurely
-insufficient surfactant production
If pre-term delivery is unavoidable/inevitable:
-glucocorticoid treatment of mother, this increases surfactant production in the fetus
When is the definitive fetal heart rate achieved by?
15 weeks
