Foetal growth and development Flashcards
Define the foetal period
0-3 weeks = pre-embryonic
3-8 weeks = embryonic
8-38 = foetal
Describe the pattern of increased foetal size, weight and body proportion during pregnancy
In the early embryonic and foetal period crown rump length rapidly increases. In the embryonic period the most significant growth is that of the placenta. There is little weight gain of the foetus. In the early foetal development there is deposition of mainly protein. In the late foetal phase there is mainly adipose deposition.
At week 9, most of CRL is the head. The body rapidly grows after this period. By term the head is approx. 1/4 of the CRL.
Describe the important events in the development of the respiratory system
Lungs develop relatively late (24 weeks) as they are not needed until birth. Bronchopulmonary tree develops during embryonic period, functional specialisations in the foetal period. Lung development has major implications for premature viability.
8-16: Pseudo glandular stage; ducts form within bronchopulmonary segments. Later become bronchioles.
16-26: Canalicular stage; ducts from before start to bud. There is more vasculature and some terminal sacs
26-terms: terminal sac stage; terminal sacs bud from bronchioles. There is differentiation of pneumocytes into type one (gas) and type two (surfactant)
term-8 years: alveolar; most alveol formed post natally. A foetus may appear to be breathing; this is conditioning of relevant muscles. Alveoli are fluid filled.
Describe the important events in the development of the nervous system
The nervous system is the first to begin but the last to finish development
16 w - corticospinal tract and cerebellar development
20 w - myelination begins in spinal cord
28 w - characteristic gyri and sulci as cerebellar hemispheres grow larger than the skull
36w- myelination begins in the brain
Accounts for 12% body weight at birth. Hearing and taste develop well before vision, which is still immature at birth. Movement can be seen on USS at 8 weeks, maternal awareness generally starts around 17.
Describe the important events in the development of the cardiovascular system
A series of shunts in place to ensure that blood is circulated around the foetus and away from the lungs. The definitive foetal heart rate is established around week 15. Foetal bradychardia is associated with foetal demise. Heart rate variability is a good indicator of healthy control systems and foetal health.
Describe the important events in the development of the urinary system
Ascent of the kidneys is completed by week 10. The metanephros, the foetal kidney, is also functional by this time. Histological differentiation is complete by 8 months. Foetal urine is the main contributor to amniotic fluid (bladder voids every 40-69 minutes)
Describe factors which influence the viability of the preterm neonate
- threshold of viability; only viable when lungs have reached terminal sac stage (around 24 weeks)
- brain maturity i.e. for breathing reflexes
- respiratory distress syndrome due to immaturity of type two pneumocytes and lack of surfactant secretion. If predictable then maternal tx with glucocorticoids.
Describe in outline the techniques used to assess foetal development
- USS
- Doppler USS
- Non Stress Test (NST) monitors heart rate
- Foetal movements kick chart
- Foetal growth measured. If lower than 10th percentile for gestational age then underweight. May be due to placental problems. Can be symmetrical or asymmetrical growth restrictions.
- symphasis to fundal height. However, twins, extra amniotic fluid or adiposity can cause errors.
How can amniotic fluid levels be a marker of disease?
oligohydramnios: too little, may be due to:
- placental insufficiency
- impaired foetal kidneys
- pre-eclampsia
polyhydramnios: too much, may be due to:
- inability to swallow
- blind ended oesophagus
- neuro - unable to coordinate swallow
What are the effects of poor maternal nutrition?
- early embryo/ foetal: neural tube defects
- late foetal: asymmetrical growth restrictions and oligohydramnios
Describe the foetal circulation and changes which are acquired at birth
- oxygenated blood enters through the umbilical vein and bypasses the liver via the DUCTUS VENOSUS
- oxygenated blood passes from the RA to LA via FORAMEN OVALE
- blood passes from pulmonary artery to aorta via the DUCTUS ARTERIOUSUS
- deoxygenated blood returns via the TWO umbilical arteries.
- Resistance in the lungs is high due to HYPOXIC PULMONARY VASOCONSTRICTION.
- after the first breath this hypoxia goes, and the lungs dilate, reducing the pressure. This increases the pressure in the LA and closes the foramen ovale. Decreased prostaglandins leads to closure of the ductus arteriosus and umbilical artery.
Describe the processes in the control of amniotic fluid volume and composition
Amniotic fluid is essentially a shock absorber and makes sure the foetus does not dehydrate. It is around 10ml at 8 weeks, rising to 1L at 38 weeks and falling to 300ml at 42 weeks. In early pregnancy it is formed from maternal fluid but this is taken over by baby later in foetal period. There is constant turnover.
Describe the function of the foetal kidneys
Functional foetal kidney is the metanephros
@25 weeks produces 100ml hypotonic urine a day
@ term this rises to 500ml a day
enters amniotic fluid which is drunk constantly. electrolytes are absorbed whilst debris remains in GIT.
This debris is known as meconium (first bowel movement)
