9. Fetal Growth & Development Flashcards
Describe the pattern of increase in fetal size, weight & body proportion
Size: crown-rump length
Increases rapidly in pre-embryonic, embryonic & early fetal periods
Weight: slow at first then increases in mid & late fetal periods
Embryo - placental growth most significant
Early fetus - weight gain related to protein deposition
Late fetus - weight gain relates to adipose deposition (metab, thermoregulation)
Body proportion: changes dramatically in fetal period
Early - head 1/2 CRL; brain takes priority
Birth - head 1/4 CRL; body growth accelerates
Describe the stages of respiratory system development in the fetal period
Pseudo glandular stage (weeks 8-16):
Duct sys begins to form within bronchopulmonary segments
Canalicular stage (weeks 16-26): Formation respiratory bronchioles More vascular but no gas exchange
Terminal sac stage (weeks 26-term):
Terminal sacs bud from respiratory bronchioles
Some primitive alveoli
Differentiation of pneumocytes (type 1 - gas exchange, type 2 - surfactant production)
Alveolar period (late fetal - 8yrs): 95% of alveoli formed post natally
Describe the key features of nervous system development in the fetus
1st to begin dev last to finish
Corticospinal tracts (begin in 4th month): required for coordinated voluntary movements
Myelination of brain (begins in 9th month)
No movement until week 8
Describe the important structural & functional changes that occur in the nervous system in the fetal period
Cerebral hemisphere becomes largest part of brain
Histological differentiation of cortex in cerebrum & cerebellum
Formation & Myelination of nuclei & tracts
Relative growth of spinal cord & vertebral column
Describe the key features of sensory & motor system development in the fetal period
Hearing & taste mature before vision
Quickening: fetal movements -
Seen on USS by week 8
Maternal awareness from week 17
Describe the key features of CVS development in the fetal period
Arranged to ensure oxygenated blood collected by umbilical vein @ placenta circulated around fetus
Definitive fetal HR achieved around 15 weeks
Fetal bradycardia associated with fetal demise
Describe the key features of kidney development in the urinary system of the fetus
(Functional embryonic kidney = mesonephros)
Ascent of kidneys complete by week 10 - fetal kidney function begins
8 months: histological differentiation of cortex & medulla almost complete
Fetal urine major contributor to amniotic fold vol
Fetal kidney fun to not necessary for survival in pregnancy
But without it = oligohydraminos
Describe the key features of bladder development in the urinary system of the fetus
Lies in abdominal cavity in fetus & infant
Urine emptied into amniotic fluid to be swallowed by fetus
Bladder fills & empties every 40-60mins
Describe 3 factors which influence viability of the pre-term neonate
Threshold of viability:
Only possible once lungs entered terminal sac stage (>24 weeks)
Brain development:
Sufficiently mature to control body functions e.g. Breathing
Respiratory distress syndrome (hyaline disease of newborn):
Insufficient surfactant production
Treatment: glucocorticoid to mother (increases surfactant production by type II pneumocytes in fetus)
Describe the techniques used to assess fetal development
Ultrasound scan (7-13 weeks - dating, 20 weeks - anomalies)
3 or 4D USS (complementary tool)
Doppler ultrasound
Non-stress test (monitors HR changes assoc with fetal mvmt)
Biophysical profiles (mvmt, breathing, tone, amniotic fluid vol, NST)
Fetal movements kick chart
Define the parameters for fetal growth restriction
Distinguish symmetrical vs asymmetrical growth restriction
Describe techniques used to estimate fetal age
Duration of pregnancy:
Fertilisation age
Age since 1st day LMP + 2 weeks
Developmental criteria: Crown-Rump Length (7-13 weeks) Biparietal diameter of head (T2/3 in combination with foot length & abdominal circumference) Weight are delivery Appearance after delivery
Symphysis-Fundal height
(But can be affected by no of fetuses, vol of amniotic fluid, lie of fetus)
Daily rhythms:
HR, breathing, activity
Amniotic fluid vol
Quickening:
maternal awareness of fetal movements from week 17 (or 20 for 1st pregnancy)
Define:
Oligohydraminos
Polyhydraminos
Too little amniotic fluid:
May be idiopathic
May be due to pre-eclampsia, placental insufficiency, fetal renal impairment
Too much amniotic fluid:
May be due to fetal abnormality (e.g. Inability to swallow - structural/neurological)
What are the parameters for classifying birth weight
4500g = macrosomia
Describe the effects on the fetus of poor nutrition during early & late pregnancy
Early:
Neural tube defects (e.g. diGeorge syndrome)
Late:
Asymmetrical growth restriction (subsequent oligohydraminos)
Describe the fetal circulation before birth
Blood flows L->R in heart
Blood delivered to hepatic portal vein (as if returning from gut)
Oxygenated blood enters fetus via umbilical vein from placenta
Bypasses liver via Ductus Venosus
Passes from RA to LA via foramen ovale
Passes from pulmonary artery to aorta via Ductus Arteriosus
Passes from aorta to carotids & returns to SVC
deoxygenated blood returns to placenta via 2 umbilical veins (?)
Resistance in lungs high: hypoxic pulmonary vasoconstriction
Describe the changes to circulation at birth
Infant takes 1st breath
Removal of hypoxic pulmonary vasoconstriction
Reduces resistance in lungs
Greater venous return to LA:
Pressure LA > RA; closure foramen ovale
Increased BP, increased smooth musc contraction; DA closes
Increased O2 saturation of blood & decreased prostaglandins (plants removed):
Constriction DA
Constriction umbilical artery
Stasis of blood in umbilical vein & DV:
Clotting of blood
Closure due to subsequent fibrosis
Describe the contents of amniotic fluid
Cells from fetus & amnion
Variety of proteins
Describe the volume of amniotic fluid:
At 8 weeks
At 38 weeks
At 42 weeks
10ml
1 L
300ml
What is the role of amniotic fluid
Mechanical protection (shock absorber)
Moist environment so fetus doesn’t dehydrate
How is amniotic fluid turned over in early & late pregnancy
Early:
Formed from maternal fluids
fetal extra cellular fluid by diffusion across non-keratinised skin
Late:
Turnover via fetus
What is the function of fetal kidneys (metanephros)
Produce fetal urine (hypotonic: lower osmotic strength than other bloody fluids)
Fetus swallows amniotic fluid, absorbing water & electrolytes
Debris accumulates in fetal gut (meconium)
How is bilirubin processed in the fetus
Fetus breaks down rbc's in speed But Fetus can't conjugate bilirubin: Crosses placenta (are accumulating; more bilirubin in fetus than mother, setting up conc gradient) Excreted by mother
Neonate may become jaundiced if conjugation not established quickly
Exposure to light stimulates liver to begin conjugation
Describe oxygen transport in fetal blood
O2 diffuses across placenta from maternal blood across thin barrier:
Driving factor = conc gradient
(placenta has low diffusion resistance; umbilical venous pO2 lower than mother)
Increasing pO2 content of fetal blood:
Fetus has diff Hb without beta chains; better in lower partial pressures
Higher affinity of fetal Hb = takes up more O2
Double Bohr effect (in maternal & fetal blood):
Increase in pCO2 (or H+) = Hb looses affinity for O2 & releases more
CO2 transfer:
Also dependent on partial pressure gradients
But fetus can’t tolerate high pCO2
Maternal pCO2 lowered by hyperventilation, stimulated by progesterone
Define:
Pre embryonic period
Embryonic period
Fetal period
Weeks 8-38
Growth & physiological maturation of structures created during embryonic period (weeks 2-8)
Preparation for transition into independent life
