S7 Fetal physiology

Question 1

describe oxygen transport in foetal blood

Answer 1

Gas exchange : diffusion barrier decreased during pregnancy, maternal P02 only slightly increases so foetal p02 must be lower than maternal.

Question 2

what is foetal 02 content increased by ?

Answer 2

foetal haemoglobin : predominant form from week 12, 2 alpha and 2 beta subunits, greater 02 affinity as doesnt bind to 2,3 - DPG as effectively as HbA
Higher Hb(18g per dl)
Foetal haemtocrit
More maternal production of 2,3 -DPG

• get a double bohr effect : increased speed of 02 transfer : when C02 enters intervillous blood get lower pH so reduced affinity of Hb for 02 ; as CO2 is lost, pH increases so Bohr effect increases affinity of Hb for 02

Question 3

describe C02 transfer in fetal physiology

Answer 3

maternal physiology adaptation to pregnancy
progesterone- driven hyperventilation
hence lower pCO2 in maternal blood
double haldane effect - feotus gives up and accepts 02/C02 so no alterations in local pH

Question 4

describe foetal circulation

Answer 4

receive oxygenated blood from mother via placenta in umbilical vein by pass the non-functional lungs return to placenta via the umbilical arteries. Small amount of blood still goes from RA to RV so muscles dont atrophy and also to lungs for their growth and development

Question 5

what is the ductus venosus ?

Answer 5

shunt needed to bypasses liver into IVC —> RA, to maintain saturation

Question 6

what is ductus arteriosus

Answer 6

Pulmonary trunk —> aorta , minimises drop on 02 saturations ( as collapsed lungs have high resistance to flow )

Question 7

what does the aorta do in foetal circulation

Answer 7

oxygenated blood to brain

Question 8

what the left atrium do in foetal circulation

Answer 8

small amount of pulmonary venous return (deoxygenated), blood reached here pumped from LV to aorta, ensures heart and brain get most 02

Question 9

what is the foramen ovale ?

Answer 9

RA pressure > LA pressure so blood goes into LA (after birth LA pressure higher so FA shuts) , free border of septum secundum forms a crest - crista dividens (which prevents mixing of venous blood from brain and directs blood to foramen ovale). Creates two streams of blood flow mainly to LA but small amount to RV, mixes with deoxygenated blood from SVC to allow the growth of the ventricular muscle

Question 10

what is foetal response to hypoxia

Answer 10

more Hbf and Hb, redistribution of flow to protect heart and brain (so less to GI, kidneys)
slowing of HR to decrease 02 demand ( low P02 or high pC02 stimulates foetal chemoreceptors ; vagus stimulation activates bradycardia - unlike in adults when low 02 causes tachycardia). Chronic hypoxaemia in pregnancy can impact growth and development

Question 11

what are the hormones necessary for foetal growth

Answer 11

insulin, IGF1 and II, leptin (placental production), TGF- alpha

Question 12

describe the effects on the foetus of poor nutrition in pregnancy

Answer 12

growth restricitons : can be symmetrical (generalised, proportional) or asymmetrical (abdomen growth lags but heads normal, more common caused by malnutrition)

Question 13

describe amniotic fluid composition

Answer 13

amniotic sac encloses the foetus in fluid, protects and aids lung development
10 ml at volume at week 8, 1 litre at 38 weeks
pre w 8 is made by the transudation of fluid across the amnion and foetal skin, inhalation of AF develops lungs, while GI absorbs essential water + electrolytes. Later is mostly urine which begins to be made in W9
compostion : 98% water, also has urea, electrolytes, creatinine, glucose, vernix caseosa (wax-type thing to protect skin)

Question 14

what is meconium

Answer 14

debris accumulates in gut : from AF and intestinal secretions —> leads to meconium
meconium in AF signigies foetal distress, shouldnt be passed until after delivery as can cause foetal RDS as it is swallowed

Question 15

describe bilirubin metabolism

Answer 15

during gestation clearance of fetal bilirubin is handled by the placenta. The fetus cannot conjugate bilirubin due to the immaturity of the liver and intestinal processes so physiological jaundice is common

Question 16

describe the foetal period

Answer 16

end of W8 - term, growth and physiological maturation of the structures created during the embryonic period. Preparation for the transition to independent life after birth
pre embryonic period is 1-3 weeks, embryonic 3-8 weeks and foetal 8-38 weeks

Question 17

describe foetal growth in the foetal period

Answer 17

dominant cell growth mechanism changes. 0-20 w = hyperplasia, 20-28w = mix of hyperplasia and hypertrophy, 28 w - term - hypertrophy
foetus relies upon relatively high maternal glucose
foetal insulin secretion begins around W10

embryonic period has intense activity but growth is small. Growth and weight gain accelerates in the foetal period

Question 18

what is the crown-rump length

Answer 18

rapidly increases in pre-embryonic, embryonic and early foetal periods
Crown-rump length (CRL) is the measurement of the length of human embryos and fetuses from the top of the head (crown) to the bottom of the buttocks (rump)
at W9, the head is half of the CRL but after this, the body lengthens and limbs grow more.

Question 19

describe weight gain in a fetus

Answer 19

weight gain is slow at first but increases rapidly in mid-late foetal period
embryo: morphogenesis and differentiation, little weight gain, placental growth is the main thing
early foetus: protein deposition
late foetus: adipose deposition

Question 20

what is amniocentesis

Answer 20

sample amniotic fluid for foetal cells to diagnose conditions, can use karyotyping

Question 21

what are the techniques used to assess foetal development

Answer 21

amniocentesis
foetal movements
Symphysis-fundal height
Ultrasound scan

Question 22

describe how the symphysis - fundal height is used to assess foetal development

Answer 22

distance in cm from pubic symphysis to fundus of uterus e.g 20 cm at 20W. 36cm at 36W or in relation to other structures. Lag of >4cm indicates foetal growth restriction

Question 23

how is an USS used to assess foetal development

Answer 23

safe, can predict weight, appearance, number, age, rule out ectopics
routinely carried out at approx 20 weeks as this is when everything in the foetus has developed large enough to see
measurements include CRL (between 7-13 weeks for dating of pregnancy), scan in T1 also used to check location, number, viability. Biparietal diameter (distance between the parietal bones, used to date in T2 and T3), abdominal circumference and femur length, 3D/4D USS

Question 24

why is dating important during pregnancy

Answer 24

as babies can have low birth weight as they are premature or they are constitutionally small (small mother will have a small baby) or they have suffered growth restriction (associated with neonatal morbidity and mortality)
late menstrual period is used to estimate foetal age but can be inaccurate due to irregular periods

Question 25

what is the normal birth weight

Answer 25

3500 g is average
<2500g suggest growth restriction
>4500 g is macrosomia (from maternal diabetes)

Question 26

describe the development of fetus respiratory system

Answer 26

lungs develop late, the embryo develops bronchopulmonary tree
around W4, respiratory diverticulum grows from the foregut ventral wall to make the tracheoesophageal septum
W8-W16 = pseudo - glandular stage. duct system forms in the bronchopulmonary segments - bronchioles. not viable, no air sacs
W16-26 - canalicular stage - budding from bronchioles into respiratory bronchioles
W26 - term = terminal sac stage, viable. Terminal sacs bud from respiratory bronchioles, type 1 and type 2 pneumocytes differentiate to produce surfactant
birth - 8y = alveolar period. 95 % of alveoli from now
During T2 and T3 gas exchange is at the placenta but the lungs prepared to take over at birth, breathing movements condition the respiratory musculature, amniotic fluid-filled lungs promote the development

Question 27

describe the development of fetus CVS ?

Answer 27

definitive foetal HR achieved at 15 w and ensures oxygenated blood is circulated around the placenta by the umbilical vein

Question 28

describe the development of fetus urinary system ?

Answer 28

kidney function begins in week 10. Kidney function not neccesary in utero but without it can oligohydramnios (deficiency of AF due to placental insufficiency e.g renal impairment) or polyhydramnios ( too much AF due to foetal abnormality e.g inability to swallow)

Question 29

describe the development of fetus nervous system ?

Answer 29

first to begin and last to finish development. In week 16 ( the corticospinal tract needed for coordinated voluntary develop. In week 36, brain myelination begins. at 15 w foetus withdraws from pain but thalamo - cortical projections do not mature until w29

Question 30

describe the development of fetus sensory and motor systems

Answer 30

no movement until w 8 , then lots of movement e.g suckling, breathing. “quickening” is maternal awareness of foetal movements, from w17. Hearing and taste develop before vision

Question 31

the development of which systems have the greatest impact on viability and survival in the event of pre-term delivery ?

Answer 31

resp and CNS

Question 32

describe the factors which influence the viability of the pre-term neonate

Answer 32

breathing ability : T II pneumocytes start making surfactant around W20 but not significantly increased until W30, not viable before 24 weeks because of this reason

Question 33

what is the respiratory distress syndrome

Answer 33

lack of surfactant in premature babies; surface tension higher , inspiration more difficult. if preterm delivery is known in advance can give glucocorticoids to increase foetal surfactant

Question 34

when does myelination begin in the spinal cord

Answer 34

20

Question 35

when do gyri and sulci appear ?

Answer 35

28 as cerebellar hemispheres grow larger than the skull