Maternal-Foetal and Placental Physiology

Question 1

What is the primary metabolic substrate for placental metabolism?

Answer 1

Glucose is the single primary metabolic substrate for placental metabolism

Question 2

How is glucose transferred across the placenta?

Answer 2

Facilitated diffusion

Question 3

How are amino acids transferred across the placenta?

Answer 3

Active transport: higher concentration in foetus than in the mother

Question 4

What does the placenta produce?

Answer 4

Oestrogen, progesterone, hCG and hPL

Question 5

Where does oxygenation of foetal blood occur?

Answer 5

Placenta, NOT lungs

Question 6

What is the pathway of oxygen blood to the foetus?

Answer 6

Oxygenated blood carried from placenta to the foetus through the umbilical vein, which enters the portal system of the foetus and branches to left lobe of liver. another branch joins blood flow from portal vein to R lobe of liver.

Question 7

What does the umbilical vein become?

Answer 7

Origin of the ductus venosus

Question 8

What happens to blood passing through the foetal right ventricle?

Answer 8

• Less than 10% to pulmonary vasculature

- Majority shunted through ductus arteriosus and descending aorta

Question 9

How does umbilical blood flow relate to foetal size?

Answer 9

Umbilical blood flow represents 40% of combined output of both foetal ventricles. Proportional to foetal growth in last half of pregnancy (~300mL/mg per minute).
i.e. relatively constant, normalised to foetal weight.

Question 10

What is foetal Hb comprised of?

Answer 10

HbF = a2, y2

Question 11

What is the key physiologic difference between HbA and HbF?

Answer 11

At any given oxygen tension, HbF has higher oxygen affinity and oxygen saturation than HbA.

Question 12

Describe O2 / CO2 exchange between mother and foetus?

Answer 12

• Maternal respiratory alkalosis facilitates transfer of CO2 from foetus
• Loss of CO2 from foetus causes rise in foetal blood pH, shifting foetal oxygen dissociation curve to left and increasing O2 binding affinity.

Question 13

Describe the foetal renal system

Answer 13

• Functional in 2nd T
• Produces dilute, hypotonic urine
• Urine production varies with size (400 - 1200mL / day)
• Primary source of amniotic fluid by middle of 2nd T

Question 14

Describe the foetal liver

Answer 14

• Slow to mature
• Capacity for glycogen synthesis and bilirubin conjugation increases with gestational age (so bilirubin primarily eliminated through placenta)
• Coag factor synthesis deficient, may be attenuated in newborn due to VitK deficiency

Question 15

How does the foetal thryoid rely on the mother?

Answer 15

• Mother only primary source of T3/T4 prior to 24 - 28 weeks
• Placenta does not transport TSH
• Gland develops without direct influence from mother, functional by end of first trimester

Question 16

What does testicular differentiation rely on?

Answer 16

Presence of H-Y antigen and Y chromosome

Question 17

Why does the mother not reject the antigenically dissimilar foetus?

Answer 17

Appears to be due to placenta

• maintaining separation of foetal and maternal compartments, keeping foetus from contact with maternal immune system
• production of oestrogen, progesterone, hCG, hPL locally decreasing immunity
• producing blocking and masking Abs

Question 18

What is the major foetal immunoglobulin?

Answer 18

Only maternal IgG can cross the placenta hence maternal IgG major proportion of foetal immunoglobulin

Question 19

What is the incidence of monozygotic twinning?

Answer 19

One in 250

Question 20

What are the different influences on monozygotic v dyzygotic twinning?

Answer 20

• Mono: not affected by race, heredity, age, parity or infertility agents.
• Dizygotic: affected by all agents

Question 21

When will MZ twins be dichorionic and diamniotic?

Answer 21

if there is separation of the blastocyst within the first 72h

Question 22

What will result if division of the embryo occurs between days 4 - 8?

Answer 22

Monochorionic, diamniotic

Question 23

When do conjoined twins occur?

Answer 23

MZ twins with late division after formation of the embryonic disk

Question 24

What happens to deoxygenated foetal blood?

Answer 24

Returned to placenta through the umbilical branches of the two hypogastric arteries. Umbilical arteries exit ado wall at umbilicus and continue through cord to placenta.

Question 25

Which condition increases incidence of SUA?

Answer 25

SUA increased in diabetic mothers

Question 26

What does SUA increase?

Answer 26

Risk of foetal malformations (up to 18%) usually involving cardiac and renal systems.

Question 27

Woman with appearance of spidery veins on face, arms, chest during later pregnancy. Mx?

Answer 27

• vascular spiders / angiomas common findings during pregnancy
• due to hyper oestrogenism
• no further workup / Rx; will resolve after delivery

Question 28

Why should pregnant women take iron in a multivitamin?

Answer 28

Even if not currently anaemic, amount of iron in diet and that can be mobilised insufficient to meet demands of pregnancy.
-pregnant woman with normal haematocrit at start of pregnancy will develop iron deficiency in latter part of gestation

Question 29

When do iron requirements increase significantly during pregnancy?

Answer 29

Second half of pregnancy

Question 30

Will the foetus suffer if maternal iron supplementation does not occur during pregnancy?

Answer 30

No. Placenta will transport required iron at expense of mother.

Question 31

Why is haematocrit not usually used as a parameter to determine when maternal iron supplementation is required?

Answer 31

Usually falls during pregnancy due to haematocrit expansion; therefore not reliable indicator of when to commence supplementation

Question 32

Normal changes to ureters / kidneys during pregnancy?

Answer 32

Bilateral mild hydronephrosis and hydroureter. Do not require additional workup.

Question 33

Why does hydronephrosis / hydroureter develop during pregnancy?

Answer 33

When gravid uterus rises out of the pelvis after 12 weeks gestation it presses on ureters at pelvic brim. Usually more pronounced on RHS. Hormonal effect of progesterone probably also contributes.

Question 34

Why do pregnant woman often have trace glucose in urine?

Answer 34

Increase in GFR and decreased tubular reabsorption of glucose

Question 35

What is placenta accrete?

Answer 35

When trophoblastic tissue invades superficial lining of the uterus. Placenta abnormally adherent and cannot be separated from uterine wall.

Question 36

What is a succenturiate placenta?

Answer 36

One or more smaller accessory lobes located in the membrane at a distance from the main placenta. Retained succenturiate lobe may cause uterine atony andPPH

Question 37

What are common triggers for haemorrhage in placenta praevia?

Answer 37

Painless haemorrhage without warning. Often occurs:

• tearing of placental attachments during formation of the LUS in T3
• Cervical dilation during term or preterm labaour

Question 38

What are the risks for placenta praevia?

Answer 38

• Grand multiparity
• Smoking
• C section

Question 39

What is fenestrated placenta?

Answer 39

Rare. Central portion of placenta is missing.

Question 40

What is membranous placenta?

Answer 40

All foetal membranes are covered by villi and placenta develops as a thin membranous structure. AKA placenta diffusa.