Foetal physiology

Question 1

What is considered low birthweight?

Answer 1

< 2500g

Question 2

What are reasons for low birthweight?

Answer 2

Prematurity
Growth retardation (small for gestational age)

Question 3

What are the values for VLBW and ELBW?

Answer 3

<1500g and <1000g respectively

Question 4

What is the fundal height?

Answer 4

The height of the top of the uterus that is measured above the pubic symphysis.

Question 5

Why is the fundal height convenient?

Answer 5

1cm roughly corresponds to 1 week of gestation.

Question 6

What are some landmarks of the fundal height?

Answer 6

At about 12 weeks, the height is palpable above the pubic symphysis. By 20-22 weeks, fundal height is at the level of the umbilicus. By 36 weeks it is up at the sternum.

Question 7

What are some genetic fetal factors affecting growth?

Answer 7

Chromosome abnormalities (trisomy 21, Turner syndrome)
Single gene (cystic fibrosis)
Sex (males larger)
Different races
Correlation between birth weight and height/weight of parents

Question 8

What are some hormones affecting fetal growth?

Answer 8

Insulin, IFG1, IFG2
NOT growth hormone or thyroxine in-utero

Question 9

What are some infections affecting fetal growth?

Answer 9

Rubella and CMV

Question 10

What are some maternal factors affecting growth?

Answer 10

1) Maternal constraint: the size of the foetus determined by the size of the mother
2) Same mother produces offspring of similar size
3) Number of foetuses - singletons are bigger than twins or triplets
4) Maternal age - young women have smaller babies
5) Parity - the first baby is smaller
6) Smoking, heroin, cocaine abuse
7) Maternal disease (hypertension, diabetes)
8) Uterine abnormalities, site of implantation - lower placental blood supply means smaller baby

Question 11

What are some placental factors affecting fetal growth?

Answer 11

True placental insufficiency is unlikely to affect foetal weight before 7th month
Correlation between weight of fetus and placenta towards term

Question 12

Why does fetal size matter?

Answer 12

Babies with reduced weight have increased mortality and morbidity.
* Prematurity - immature organ systems
* Lack of surfactant –> respiratory distress syndrome
* Inadequate fat stores
* Inability to maintain body temperature

Question 13

What is the Barker hypothesis?

Answer 13

An inverse relationship between birth weight and adult disease. Also known as intrauterine programming.

Question 14

What does it mean when the placenta is ‘haemochorial’?

Answer 14

The chorion is in direct contact with the maternal blood.

Question 15

What are the descriptors of the placenta?

Answer 15

Discoid shape
15-20cm in diameter
3cm thick
1/6 of fetal weight at term
200 lobules

Question 16

What are some functions of the placenta?

Answer 16

1) Secretion of hormones (progesterone, oestrogen, hCG and hPL)
2) Exchange between mother and foetus - heat, gases, nutrients, wastes etc.
3) Metabolism
4) Immunological barrier

Question 17

How does the syncytiotrophoblast surrounding the placenta change?

Answer 17

Over gestation, the syncytiotrophoblast surrounding villi gets smaller, allowing more exchange to occur. This means placental conductivity increases over gestational age.

Question 18

What is the composition of fetal calories?

Answer 18

1/2 glucose, 1/4 amino acids, 1/4 lactate

Question 19

How is glucose transferred to the foetus?

Answer 19

Facilitated diffusion (GLUT1 protein)

Question 20

How are amino acids transported?

Answer 20

Mostly actively transported through several stereospecific transporters

Question 21

How is water transported?

Answer 21

Freely crosses placenta and membranes down osmotic gradients

Question 22

Why are fatty acids important?

Answer 22

Important for cell membranes, brain development and surfactant

Question 23

What is ‘parallel pumps’ in fetal circulation?

Answer 23

Both R and L ventricles supply blood to the main part of the body with the R ventricle being the dominant one.

Question 24

What are some key features of fetal circulation?

Answer 24

1) High resistance pulmonary bed
2) Gas exchange occurs at the placenta
3) Mixing of oxygenated and deoxygenated blood
4) Placenta, umbilical vessels, shunts

Question 25

How much of the combined cardiac output does the placenta receive?

Answer 25

At least 40%

Question 26

What is the blood supply to the placenta?

Answer 26

Umbilical arteries branch from internal iliac arteries, carry deoxygenated blood to the placenta.

Question 27

How does blood return from the placenta?

Answer 27

Through one umbilical vein which carries oxygenated blood (sat 80-85%) from the placenta back to the fetal body

Question 28

Where do the three shunts occur?

Answer 28

Ductus venosus, ductus arteriosus and foramen ovale.

Question 29

What is the ductus venosus?

Answer 29

Connects the abdominal umbilical vein with inferior vena cava, near the entrance to the heart. This allows for it to bypass the liver (55%).

Question 30

What is the role of the foramen ovale?

Answer 30

The foramen ovale allows the highly oxygenated blood which has come through the ductus venosus and gone through the posterior left stream of the vena cava to go straight into the left side of the heart.

Question 31

What is the purpose of the ductus arteriosus?

Answer 31

Connects the pulmonary trunk to the descending aorta. Bypasses lungs - which means that the poorly oxygenated blood being pumped out of the right side of the ventricle - some goes to the lungs and some to the rest of the body.

Question 32

Summarise the process of fetal circulation.

Answer 32

Umbilical arteries (with deoxygenated blood) --> blood oxygenated in the placenta --> umbilical vein (contains oxygenated blood 80-85 Sat) --> most bypasses liver via ductus venosus --> blood from DV streams up posterior wall of IVC --> most of this well oxygenated blood is directed posteriorly across foramen ovale into left atrium --> mixes with a small amount of blood returning from lungs and then enters left ventricle --> blood entering the ascending aorta from the LV has 60-65% Sat --> 10-15% continues in the aorta --> blood from the poorly oxygenated stream plus blood from the SVC enters the right atrium and then the right ventricle --> blood leaving the right ventricle has a Sat 50-52% --> from the pulmonary artery about 7% goes to lungs --> the remainder leaves the pulmonary artery via the DA which enters the aorta below the exit of the left subclavian artery --> blood in the descending aorta has Sat 55-58% --> this blood supplies the rest of the body including the placenta

Question 33

What are some other differences between fetal and adult circulation?

Answer 33

1) Arterial oxygen tension varies 2) Combined cardiac output high 3) Blood pressure low 4) Heart rate fast 5) Kidneys only 2-3% of CCO

Question 34

How does the fetus respond to hypoxia?

Answer 34

1) Increase in BP - vasoconstricts the 'less important' parts of its body e.g gut, lungs, limbs --> more blood to placenta through ductus venosus and to brain, heart and adrenals 2) Decrease in HR - caused by baroreflex. HR that doesn't return to normal can be indicator of fetal distress and lead to caesarean section

Question 35

What is amniotic fluid formed by?

Answer 35

1) Foetal urine - large dilute volume 2) Lung liquid - clear, colourless, isosmotic with plasma, high chloride content, important for lung growth

Question 36

How does fluid leave the amniotic cavity?

Answer 36

1) Swallowing - associated with foetal breathing movement and important for gut development 2) Equilibration across membrane with maternal and foetal plasma --> intramembranous = movement to the fetal plasma --> transmembranous = movement to the mother

Question 37

What are some functions of amniotic fluid?

Answer 37

1) Insulation against trauma 2) Allows symmetrical growth 3) Stops amnion sticking to fetus 4) Important for lung development 5) Enable fetal limb movement, respiratory movements, swallowing 6) Helps maintain constant body temperature 7) Potential source of fluid and electrolytes 8) Immunoprotection

Question 38

What are some abnormalities of amniotic fluid?

Answer 38

1) Oligohydramnios = too little fluid 2) Polyhydramnios = too much fluid