Session 8 - Placenta and Maternal Adaptations

Question 1

Around what day is implantation on average?

Answer 1

Day 6

Question 2

What changes are required in the endometrium to prepare for implantation?

Answer 2

Pre-decidual cells are stimulated by progesterone to become decidual cells. These are important cells that counter the invasive force of the embryo.
Spiral arteries are remodelled so that a low resistance vascular bed exists. This is most important in late pregnancy. Extra villus trophoblasts displace maternal lining and line the blood vessels instead.

Question 3

What problems exist if decidualisation does not occur?

Answer 3

Can get haemorrhage due to ectopic pregnancy.
If implantation occurs at a site other than the uterus then you also get ectopic pregnancy as there are no decidual cells to counter the invasive force of the embryo so it burns itself outside of the uterus.

Question 4

What problems exists if the vascular bed is not remodelled?

Answer 4

Pre-eclampsia/eclampsia
Resistance in vascular bed is higher than it should be this results in maternal hypertension, Proteinuria, pitting oedema, seizures in eclamptic state.
Because you don’t get establishment of a good maternal-foetal circulation.

Question 5

What is ectopic pregnancy?

Answer 5

Implantation of embryo at a site other than the uterus.

Commonly the fallopian tube, however can be peritoneal or ovarian. Can be very quickly a life threatening emergency.

Question 6

What is placenta praevia?

Answer 6

When the embryo implants in the lower uterine segment which can block the cervix. This may cause haemorrhage during pregnancy and required a C-section delivery.

Question 7

What are the aims of implantation?

Answer 7

Establish basic unit of exchange
Anchor the placenta
Establish maternal blood flow with the placenta

Question 8

Describe the process of implantation:

Answer 8

Trophoblasts have an invasive property which embed the blastocyst into the posterior uterine wall ideally. The trophoblasts invade through the uterine epithelium and implants in the stroma. The trophoblasts differentiate and form an outer later of syncitiotrophoblasts and cytotrophoblasts.

Question 9

Describe the development of the villi:

Answer 9

Primary villi are early finger-link projections of the trophoblast.
Secondary villi - mesenchyme invade the core
Tertiary Villi - Fetal vessels invade the mesenchyme core

Question 10

Describe the development of the metal membranes:

Answer 10

The chorion complete surrounds the amnion and had villi all around it. As the amnion increases in size and the decidua capsularis and the decidua parietalis move closer together the villi recede to form a disc shape.
The amniochorionic membrane fuses to the decidua parietalis.

Question 11

Describe the structure of the placenta:

Answer 11

Fetal portion - Formed by chorion frondsum and bordered by the chorionic plate - arteries and veins returning from chorionic villi
Maternal portion - Formed by the decidua basalis and decimal plate which is intimately incorporated into placenta.
Between chorionic and decidual plates are intervillus spaces which are filled with maternal blood.

Question 12

Describe the arrangement of blood vessels in the placenta:

Answer 12

Endometrial arteries form spiral arteries which pierce the decidual plate and supply the intervillus spaces. The villi contain fetal capillaries that join to form an umbilical vein and two umbilical arteries. The umbilical vein sends oxygenated blood to the foetus and the umbilical arteries drain deoxygenated blood away from the foetus.

Question 13

What are some of the differences between first trimester and term placenta?

Answer 13

The placental barrier to diffusion is pretty thick in first trimester but thins in term placenta. In the first trimester villi have a complete cytotrophoblast layer underneath syncytiotrophoblast. This is lost in a term placenta. The surface area for exchange is massively increased in a term placenta.

Question 14

What factors influence the diffusion of substances across the placenta?

Answer 14

Concentration gradient - steeper the concentration gradient the more diffusion
Barrier to diffusion - Placental membrane gradually thins throughout pregnancy as the demand of the foetus increases.
Diffusion distance - Hamomonochorial

Question 15

What substances are transported across the placenta and how?

Answer 15

Simple diffusion: water, electrolytes, urea and uric acid, Gases - O2 and CO2 - stores of O2 are limited so flow but be high at all times.
Facilitated diffusion: glucose
Active transport - Amino acids, Iron, Vitamins
Receptor Mediated Pinocytosis - Immunoglobulins

Question 16

What substances are harmful to the foetus but can still cross the placenta?

Answer 16

Teratogens such as thalidomide, alcohol, therapeutic drugs, illegal drugs, maternal smoking.
Pathogens such as - toxoplasmosis, rubella, varicella, CMV, treponoma pallidum.

Question 17

What is the endocrine function of the placenta?

Answer 17

Production of protein hormones
- Human chorionic gonadotrophin hCG (produced during first two months of pregnancy, Supports secretory function of corpus luteum, produced by syncytiotrophoblast.
- Human chorionic somatommotrophin hCS (Influences maternal metabolism but increasing the availability of glucose to the foetus)
- Human Chorionic Thyrotrophin
- Human Chorionic Corticotrophin
Steroid Hormones
- Progesterone (placenta takes over production from corpus luteum at around week 11, also increases maternal appetite)
- Oestrogen

Question 18

Describe the hormonal basis of pregnancy testing?

Answer 18

Test for presence of human chorionic gonadotrophin which is produced in the first two months of pregnancy.
Produced by syncytiotrophoblast cells.
Excreted in maternal urine.

Question 19

Describe passive immunity in the case of a foetus:

Answer 19

The mother can pass on immunoglobulins in the form of IgG by receptor mediated pinocytosis until fetal plasma immunoglobulin exceeds levels in maternal plasma. This allows the baby to be protected in the neonatal phase until the babies own immune system starts to produce antibodies.

Question 20

What are some of the problems of placental immunoglobulin transfer?

Answer 20

Can cause haemolytic disease in the newborn if there is rhesus group incompatibility of mother and father. This only becomes a problem in a second pregnancy as the mother will already have the antibodies against the antigen, having previously been sensitised. If the IgG crosses the placenta it will attack fetal RBC’s. This can be prevented by prophylactic treatment.

Question 21

What physiological changes to the cardiovascular system are experienced by mothers during pregnancy?

Answer 21

Blood volume increase
CO increase
Stroke volume increase
HR increase
BP is usually normal or can be low - hypertension usually indicates a problem - although CO increases TPR decreases as progesterone relaxes smooth muscle systemically.
Hypotension in T1 and T2 is due to the effect of progesterone on the TPR
T3 - aortocaval compression by gravid uterus can cause reduced return to the heart. Although blood pressure should be normal at this point.

Question 22

What physiological changes to the urinary system are experienced by mothers during pregnancy?

Answer 22

Renal plasma flow increases
Glomerular filtration rate increases
(creatine and plasma clearance increase and plasma urea, uric acid, bicarbonate and creatinine decrease)
- functional renal reserve decreases as GFR increases.
Progesteron causing smooth muscle relaxation can lead to urinary stasis, hydrometer, UTIS and pyelonephritis.

Question 23

What physiological changes to the respiratory system are experienced by mothers during pregnancy?

Answer 23

Diaphragm is displaced
A-P and transverse diameters of thorax increases
Decreased functional residual capacity
Vital capacity unchanged
Tidal Volume increases
Respiratory minute volume and alveolar ventilation rate increased.
HOWEVER RR unchanged
O2 consumption increases 20%
Progesterone leads to hyperventilation as the mother needs to blow off extra CO2 generated by foetus, this leads to respiratory alkalosis which the kidneys compensate for by reabsorbing less bicarb.

Question 24

What physiological changes to carbohydrate metabolism are experienced by mothers during pregnancy?

Answer 24

Fat is played down during the first trimester so that stores are available in the third trimester when the foetus is more metabolically demanding. This is caused by progesterone stimulating appetite. Human placental lactogen and oestrogen stimulate a prolactin release which generates a maternal resistance to Insulin. Maternal glucose use decreases stimulating gluconeogenesis and more glucose is available to the foetus. Fasting blood glucose in pregnancy is decreased and there is an increase in post-prandial blood glucose.
In later pregnancy the mothers needs are met by metabolising fatty acids.

Question 25

What is gestational diabetes?

Answer 25

A carbohydrate intolerance first recognised in pregnancy and does not persist after delivery. This can result in a macrocosmic foetus, stillbirth and increased risk of congenital defects.
Gestational diabetes is caused when the pancreas is unable to keep up with the increasing demand for Insulin which is usually achieved by beta cell hyperplasia and hypertrophy. This leads to a loss of control of metabolism, blood glucose increase, therefore diabetes.

Question 26

What physiological changes to lipid metabolism are experienced by mothers during pregnancy

Answer 26

Increases in lipolysis in T2 onwards.
There is an increase in concentration of free fatty acids when fasting. Fatty acid forms the substrate of maternal metabolism leaving glucose for the foetus.
Increased use of fatty acid can lead to ketoacidosis.

Question 27

What physiological changes to the thyroid are experienced by mothers during pregnancy?

Answer 27

TBG production increased.
T3/T4 increased
Free T4 in normal range as binding protein and hormone are both increased.
hCG stimulates T3/T4 production, therefore TSH can be reduced due to negative feedback.

Question 28

What physiological changes to the GI system are experienced by mothers during pregnancy?

Answer 28

Alteration of positions of the viscera - appendix moves for RLQ to LUQ as uterus enlarges.
Progesterones affect on SM means that there is:
- Delayed emptying
- Biliary tract stasis
- Increased risk of pancreatitis as pancrease builds up.

Question 29

What physiological changes to the blood are experienced by mothers during pregnancy?

Answer 29

Pregnancy is a pro-thrombotic state
High amount of fibrin deposition at implantation site - increasing fibrinogen and other clotting factors, reduced fibrinolysis.
Stasis - venodilation
Cannot give warfarin - crosses placenta and is teratogenic.
Physiological anaemia - mismatch between plasma volume and haematocrit - Plasma volume and RBC volume increases but RBC not so much.

Question 30

What physiological changes to the immune system are experienced by mothers during pregnancy?

Answer 30

Non- specific suppression of immune system at materno-fetal interface - otherwise immune system would attack foetus as it is foreign - genetically different
Graves and Hashimoto’s - antibodies cross and can end up destroying fetal thyroid.

Question 31

What factors are examined in ante-natal screening?

Answer 31

History and Examination - check for risk factors e.g gestational diabetes
Blood test - Blood group, haemoglobin, infection
Urinalysis - Protein