Physiology Of Pregnancy

Question 1

Timeline of a pregnancy

Answer 1

Trimester 1 = 1-12 weeks growth and development of the foetus and placenta
Trimester 2 = 13-26 weeks continuing growth and development of the placenta
Trimester 3 = 27-40 weeks rapid growth of the foetus

Normal = 37-42 weeks

Question 2

When does pregnancy start

Answer 2

Starts at last menstrual period
Conception is week 3 and implantation is week 4
Can’t be week 1 or 2 due to it being the follicular phase

Equation to calculated estimated delivery date = add 1 year to last menstrual period and 7 days. Subtract 3 months

Question 3

Implantation of the embryo

Answer 3

1) embryo hatches from zona pellucida
2) there is alteration is glycoprotein mucins that line the epithelial cells of the uterus
3) this altercation is triggered by proteases secreted from the embryo
4) trophectoderm have villi that interdigitate with villi of endometrial epithelia
5) this causes interaction and triggers implantation

Question 4

What post implantation changes occur to the trophoectoderm

Answer 4

Differentiate into two types of trophoblast cells:
1) syncytial trophoblasts = multinucleated cells formed under fusion event
2) cytotrophoblasts = retain individual cellularity and form important part of the placenta

This occurs within the wall of the uterine lining and after the epithelium has grown over the conceptus

Question 5

What happens to the foetus in each trimester

Answer 5

T1 - fertilisation, implantation, initial development, placentation

T2 = nervous system, hair, spine straightens, pain, proportions change

T3 = growth, fat deposition, brain growth, blood cells, lung development

Question 6

What happens to the mother in each trimester

Answer 6

T1 = weight gain and nausea

T2 = placental growth, uterus rises, hypervolemia, cardiac remodelling and breast remodelling

T3 = Braxton hicks, tiredness, restricted breathing and lactation

Question 7

Maternal changes

Answer 7

Steady increase in heart rate
Not any change in blood pressure
Steady increase in blood volume from week 6 onwards up to 32

Question 8

Foetal changes

Answer 8

Rapid increase in heart rate in first trimester which then stabilises at 150bpm for rest

Increase in foetal size (about 100g in 1st trimester)

Question 9

What is histotropic support and what is it replaced with later on

Answer 9

Now after implantation embryo can be supported by glands of the uterus

As synctiotrophoblast digest the lining of the uterus and stroma. As it does this it forms gaps called lacunae within the syncytiotrophoblast which accumulate nutrients to support growth. As syncytiotrophoblast grows it encounters spiral blood vessels and remodels it so that they are no longer constricted and providing blood at a high pressure. It invades them so they widen and there is low pressure lake/pool flow to the lacunae where it terminates. The primary villi/stem villi originate from embryonic mesoderm and branch extensively forming tertiary villi leading to formation of foetal blood vessels/vasculature parallelly. Concept that maternal blood pooling in lacunae never mixes with foetal
This happens in fifth week
Changed from histotrophic support to haematotrophic support

Question 10

Early signs of pregnancy

Answer 10

Suppression of menses
Tender/ enlarged breasts
Fatigue
Urinary frequency
Nausea/vomiting
Constipation

Question 11

What is the amnion, chorion and yolk sac

Answer 11

Ectoderm + mesoderm = amnion

Trophoblast + mesoderm = chorion

Endoderm + mesoderm = yolk sac

Question 12

Villi classification

Answer 12

Primary when composed of solid trophoblast

Secondary when mesoderm invades the villous core

Tertiary when blood vessels penetrate the villi

Question 13

Villi development

Answer 13

wk 3 = primary stem villi penetrated by extraembryonic mesoderm

Wk 9 = tertiary stem villi lengthen forming mesenchymal villi

Wk 16 = terminal extensions reach maximum length - immature intermediate villi

Wk 32 = mature intermediate tertiary villi produce small nodule like secondary branches - terminal villi
Final structure of the villus tree

Question 14

What is the surface are of the placental villi at 28 weeks and at term

Answer 14

28 weeks = 5m^2

At term = 11m^2

Question 15

Umbilical cord and nutrient exchange

Answer 15

PO2 of maternal blood is 50mmHg whilst foetal is 30mmHg but due to foetal Hb having a higher affinity for oxygen it is able to pinch off oxygen.
Foetal Hb carries 20-50% more oxygen and its concentration is 50% greater than maternal blood

Question 16

The placenta is responsible for the transport of which substances

Answer 16

• glucose via GLUT 1,3,4 and 12 transporters
• VLDLS and chylomicrons which are metabolised by lipoprotein lipase
• free fatty acids, glycerol and cholesterol

Question 17

How does the placenta act as an endocrine organ

Answer 17

• produces hCG which maintains the corpus luteum + stimulates thyroid
• significant amount of oestrogen produced ( proliferative effects on mother and relaxes pelvic ligaments and increases elasticity of pubic symphysis
• progesterone = essential for pregnancy, causes decidualisation, increases secretions and reduces uterine contractions
• somato-mammotropin ?

Question 18

What occurs in T2

Answer 18

Growth = triple in size and increase in weight by 30X
Hypervolemia = leads to maternal glow
Fundus rises by 1cm each week
Organs become squeezed especially lungs
hCG levels fall as CL regresses and placenta takes over

Question 19

How does the action of progesterone and oestrogen increase blood volume

Answer 19

Oestrogens =
Increase angiogenesis
Increase blood flow
Increase venous distensibility (by increasing NO and decreased endothelin 1)

Increase hepatic angiotensin II
Increase renal re absorption
Increase aldosterone

All lead to retention of fluid and increase in blood volume but may also lead to oedema

Progesterone
Leads to increased vasodilation and thus decreased peripheral resistance
Increases aldosterone by x10
Increased activation of thirst centre

Question 20

Cardiac remodelling

Answer 20

Parallel increase relative to blood volume.
Heart rate and stroke volume increases by 20%
Size of heart increases by 12%

Increased blood volume leads to increased venous return and so increased atrial size

Question 21

How do the lungs adapt

Answer 21

Respiratory effect is increased as more oxygen is consumed (20% more) and also because of reduced space which displaces the rib cage upwards

So sensitivity to chemoreceptors is lowered (allows deeper breathing and increase in tidal volume)
Responsiveness to PCO2 increased

Question 22

How is the renal and urinary system remodelled

Answer 22

Kidneys enlarge ( increased excretion of waste, increased re absorption of sodium)

Ureters displaced and enlarged

Leads to decreased bladder tone which may cause urinary reflux and stasis, higher amounts of glucose and AAs in urine. All increase risk of UTIs

Question 23

What happens to the breast

Answer 23

Progesterone causes more growth of alveoli and lobules

Oestrogens stimulates growth and development of milk ducts

HPL = mimics prolactin and GH, causes breast, nipple and areola to enlargen

Question 24

Average weight gain

Answer 24

11kg

Required additional calorie intake - 100 to 300 calories extra

Question 25

Patterns of female fertility

Answer 25

With age there is reduced fertility, ovarian function, uterine function, ovarian reserve.

20% chance of conception at 30

5% at 40yrs

Question 26

Implantation of the embryo is dependent on which four things

Answer 26

Oestrogen
Leukaemia inhibitory factor
Epithelial growth factor
Epidermal growth factor