Pregnancy, Parturition & Late Foetal Development

Question 1

How would you describe Embryo-foetal growth during the first trimester?

Answer 1

Limited

Question 2

What word describes early embryo nutrition?

Answer 2

Histiotrophic

Question 3

Define Histiotrophic

Answer 3

Reliant on uterine gland secretions and breakdown of endometrial tissues

Question 4

What nutritional switch at the beginning of the second trimester allows for accelerated growth?

Answer 4

Haemotrophic support

Question 5

What happens in Haemotrophic support?

Answer 5

haemochorial-type placenta where maternal blood directly contacts the fetal membranes.
Around 12th week

Question 6

What is special about the amnion?

Answer 6

Arises from the epiblast (but does not contribute to the fetal tissues)
Forms a closed, avascular sac with the developing embryo at one end
Begins to secrete amniotic fluid from 5th week – forms a fluid filled sac that encapsulates and protects the fetus

Question 7

What is the chorion?

Answer 7

Formed from yolk sac derivatives and the trophoblast
Highly vascularized
Gives rise to chorionic villi – outgrowths of cytotrophoblast from the chorion that form the basis of the fetal side of the placenta

Question 8

What is the connecting stalk?

Answer 8

Links the developing embryo unit to the chorion.

Question 9

What is the Trophoblastic Lacunae?

Answer 9

Large spaces filled with maternal blood formed by breakdown of maternal capillaries and uterine glands
Become intervillous spaces aka maternal blood spaces

Question 10

What are the Foetal membranes?

Answer 10

Fetal membranes: extraembryonic tissues that form a tough but flexible sac encapsulates the fetus and forms the basis of the maternal-fetal interface.

Question 11

What happens when the amniotic sac expands?

Answer 11

Expansion of the amniotic sac by fluid accumulation forces the amnion into contact with the chorion, which fuse, forming the amniotic sac
Amniotic sac: 2 layers; amnion on the inside, chorion on the outside

Question 12

What is the Allantois?

Answer 12

Outgrowth of the yolk sac
Grows along the connecting stalk from embryo to chorion
Becomes coated in mesoderm and vascularizes to form the umbilical cord.

Question 13

What are the Chorionic Villi?

Answer 13

Provide substantial surface area for exchange

Finger-like extensions of the chorionic cytotrophoblast, which then undergo branching

Question 14

What are the 3 stages of Chorionic Villi development?

Answer 14

Primary: outgrowth of the cytotrophoblast and branching of these extensions
Secondary: growth of the fetal mesoderm into the primary villi
Tertiary: growth of the umbilical artery and umbilical vein into the villus mesoderm, providing vasculature.

Question 15

What is the Terminal villus Microstructure?

Answer 15

Convoluted knot of vessels and vessel dilation

Slows blood flow enabling exchange between maternal and fetal blood

Whole structure coated with trophoblast
(missing here as capillary cast)

Question 16

What are the characteristics of the villi blood vessels in Early Pregnancy?

Answer 16

150-200µm diameter, approx. 10µm trophoblast thickness between capillaries and maternal blood

Question 17

What are the characteristics of the villi blood vessels in late pregnancy?

Answer 17

villi thin to 40µm, vessels move within villi to leave only 1-2µm trophoblast separation from maternal blood.

Question 18

What are the vessels that supply blood to the foetus?

Answer 18

Uterine artery branches give rise to a network of arcuate arteries.

Radial arteries branch from arcuate arteries, and branch further to form basal arteries.

Basal arteries form spiral arteries during menstrual cycle endometrial thickening.

Question 19

What happens in Spiral artery remodelling?

Answer 19

Spiral arteries provide the maternal blood supply to the endometrium

Extra-villus trophoblast (EVT) cells coating the villi invade down into the maternal spiral arteries, forming endovascular EVT.

Endothelium and smooth muscle is broken down – EVT coats inside of vessels

Conversion: turns the spiral artery into a low pressure, high capacity conduit for maternal blood flow.

Question 20

How does oxygen get across the Placenta?

Answer 20

diffusional gradient (high maternal O2 tension, low fetal O2 tension)

Question 21

How does glucose get to the Foetus?

Answer 21

facilitated diffusion by transporters on maternal side and fetal trophoblast cells.

Question 22

How does water get to the Foetus?

Answer 22

placenta main site of exchange, though some crosses amnion-chorion. Majority by diffusion, though some local hydrostatic gradients

Question 23

How do electrolytes get to the Foetus?

Answer 23

large traffic of sodium and other electrolytes across the placenta – combination of diffusion and active energy-dependent co-transport.

Question 24

How does Calcium get to the Foetus?

Answer 24

actively transported against a concentration gradient by magnesium ATPase calcium pump

Question 25

How do Amino acids get to the Foetus?

Answer 25

reduced maternal urea excretion and active transport of amino acids to fetus

Question 26

What are the changes in the maternal circulation?

Answer 26

Maternal cardiac output increases 30% during first trimester (stroke vol & rate)

Maternal peripheral resistance decreases up to 30%

Maternal blood volume increases to 40% (near term (20-30% erythrocytes, 30-60% plasma)

Pulmonary ventilation increases 40%

Question 27

What are the changes in Placenta and Foetus circulation + consumption?

Answer 27

Placenta consumes 40-60% glucose and O2 supplied

But although fetal O2 tension is low, O2 content and saturation are similar to maternal blood.

Embryonic and fetal hemoglobins: greater affinity for O2 than maternal hemoglobin.