Pregnancy Physiology

Question 1

Stages of conception

Answer 1

Binding of sperm to zona pellucida specific cell surface glycoproteins - acrosome swells and outer membrane fuses with sperm plasma membrane
Egg cell membrane depolarises - prevents polyspermy
Cortical granules than lie just beneath egg cell membrane fuses with sperm membrane to release sperm contents
Second meiotic division of egg -> polar body

Question 2

Define cervical ripening

Answer 2

Softening of cervix occurs before labour

- occurs in response to oestrogen, relaxin and prostaglandins

Question 3

What does cervical ripening involve

Answer 3

Reduction in collagen
Increase in glycosaminoglycans
Increase in hyaluronic acid
Reduced aggregation of collagen fibres

Question 4

Role of oxytocin in labour

Answer 4

At 36 weeks increase in number of oxytocin receptors present in myometrium and decrease in relaxin and progesterone inhibition
Leads to positive feed back loop - Ferguson reflex
-brain stimulates pituitary gland to secrete oxytocin
- oxytocin carried in bloodstream to uterus
- oxytocin stimulates uterine contractions and pushes baby towards cervix
- head of baby pushes against cervix
- nerve impulses form cervix transmitted to brain

Question 5

Factors that contribute to myometrial excitability

Answer 5

Relative decrease in progesterone in relation to oestrogen

Mechanical stretching of uterus

Question 6

Stages of labour

Answer 6

1st stage - from regular contractions (3-4 every 10mins) till cervix fully dilated (10cm)
2nd stage - from fully dilated cervix till baby born
3rd stage - from baby born till placenta out

Question 7

Sections of first stage of labour

Answer 7

Contractions occur every 2-3 mins
Foetal membranes rupture
Latent phase - slow cervical dilation over several hours until cervix reaches 4cm
Active phase - faster rate of cervical dilation till 10cm
- should not last more than 16 hours

Question 8

Sections of second stage of labour

Answer 8

Passive stage - lasts until head of foetus reaches the pelvic floor
- women experiences desire to push
- rotation and flexion occurs
- lasts few mins
Active stage - women pushes in conjunction with her contractions to expel foetus

Question 9

Regulation of contractions

Answer 9

Fibres of myometrium do not fully relax following each contraction - steadily reduces uterine capacity, increasing pressure
Prostaglandins - more intracellular calcium released per action potential - increases force
Oxytocin - lowers threshold for action potentials - increases frequency

Question 10

Actions of delivery of baby

Answer 10

Engagement - head to cervix
Flexion - chin to chest
Descent - through cervix
Internal rotation - facing floor/anus
Extension - head back round pubic symphysis
External rotation - to pass shoulders

Question 11

Events during third stage of labour

Answer 11

Uterine muscles contract to compress bleed vessels supplying placenta -> shears away from uterine wall
Lasts 15 mins
500ml blood loss normal

Question 12

Control of bleeding during third stage

Answer 12

Contraction of uterus constricts blood vessels in myometrium
Pressure exerted on placental site once delivered by walls of contracted uterus
Normal blood clotting mechanism

Question 13

Changes to breasts in pregnancy

Answer 13

Hypertrophy of ductular-lobular-alveolar system

Little milk secretion during pregnancy due to high progesterone:oestrogen ratio favours growth

Question 14

Consitiuents of maternal milk

Answer 14

Colostrum - first few days of secretion from the mammary glands
- thick yellowy liquid with high concentrations of antibodies, proteins and fat soluble vitamins

Question 15

Regulation of milk production

Answer 15

Prolactin secreted by anterior pituitary gland
- controlled by dopamine from hypothalamus
- stimulated by suckling
Suckling stimulates receptors in nipple and impulses pass up brain stem and reduce dopamine secretion -> increased prolactin secretion
Suckling at one feed promotes prolactin release which causes production for next feed which accumulates in alveoli and ducts (turgor)

Question 16

Milk let down reflex

Answer 16

Babies do not suck milk out - ejected out by let-down reflex
In response to suckling oxytocin released from pituitary gland -> stimulated myoepithelial cells that surround alveoli to contract -> squeezes milk out of breast
Cry or sight of infant and preparation of breast for nursing may cause let down
Pain, embarrassment of alcohol may inhibit

Question 17

Changes to endocrine system

Answer 17

Increased oestrogen
-> increased in hepatic thyroid binding globulin -> more free T3/4 binds to TBG -> increased TSH -> increased total T3/4 - thyroxin essential for foetal development but thyroid gland not functional till 2nd trimester
Increase in human placental lactogen, prolactin and cortisol (anti-insulin hormones)
-> increased insulin resistance and reduced peripheral glucose uptake - ensures continuous glucose supply to foetus
-> increased maternal lipolysis -> increased free fatty acids - increased risk of ketoacidosis

Question 18

Changes to CVS system

Answer 18

Increased progesterone
- decrease in diastolic blood pressure - 1st and 2nd trimester
- cardiac output increases by 30-50%
Activation of RAAS
- increase in sodium levels and water retention
- increased total blood volume

Question 19

Changes to resp system

Answer 19

Upward displacement of diaphragm - increase in transverse and AP dimeters of thorax
Increased metabolic rate -> increased demand for oxygen
- increased tidal volume and minute ventilation rate
Hyperventilation due to increased CO2 production and increased resp drive by progesterone -> resp alkalosis compensated by increase renal bicarb excretion

Question 20

Changes to GI system

Answer 20

Upward displacement of stomach and increased intra-gastric pressure
-> GI reflux, N+V
Appendix moves to RUQ
Increased progesterone -> smooth muscle relaxation
-> decreased gut motility - more time for nutrient absorption -> constipation
-> relaxation of gallbladder -> increased risk of gallstones

Question 21

Changes to urinary system

Answer 21

Increased CO -> increased plasma flow -> increased GFR by 50-60% - increased renal excretion so lower urea and creatinine levels
Progesterone -> relaxation of ureter and muscles of bladder -> urinary stasis -> predisposes to UTIs and pyelonephritis

Question 22

Haematological changes

Answer 22

Increase in fibrinogen and clotting factors and decreased fibrinolysis
Progesterone -> blood stasis and venodilation
Increased risk of VTE
- cannot give warfarin as teratogen
- give LMWH
Increased plasma volume but red cell mass does not increase by as much -> dilutional anaemia

Question 23

Placenta pre-implantation

Answer 23

Development begins during implantation of blastocyst
32-64 cell blastocyst contains two differentiated cell types
- outer trophoblast cells -> placenta
- inner cell mass -> foetus and foetal membranes

Question 24

Placenta during implantation

Answer 24

On 6th day zona pellucida disintegrates and blastocyst hatches
Trophoblast cells interact with endometrial decidual epithelia to enable invasion of maternal uterine cells
Embryo secretes protases - allows deep invasion into uterine stroma
8th day trophoblast cells differentiate into
- outer multinucleated syncytiotrophoblast - erodes maternal tissue + produces hCG
- inner mononucleated cytotrophoblast - actively proliferating

Question 25

Placenta post-implantation

Answer 25

Day 9 lacunae form within syncytiotrophoblast
- erodes maternal tissues allowing maternal blood from uterine spiral arteries to enter lacunar network
Cytotrophoblast form primary chorionic villi
- penetrate and expand into surrounding syncytiotrophoblast
- in 3rd week extra-embryonic mesoderm grow into villi forming core of loose connective tissue -> secondary chorionic villi
By end of 3rd week embryonic vessels begin to form in embryonic mesoderm -> tertiary chorionic villi
Cytotrophoblast cells from tertiary villi grow towards decidua basalis of maternal uterus and spread to form cytotrophoblastic shell
- villi connected to decidua basalis through cytotrophoblastic shell = anchoring villi
Villi growing outward within intervillous space stem from anchoring villi = branching villi
- provide surface area for exchange

Question 26

Establishment of circulation in placenta

Answer 26

Maternal spiral arteries undergo remodelling to produce low resistance high blood flow conditions
- cytotrophoblast cells invade maternal spiral arteries and replace spiral endothelium

Question 27

Placental thickness

Answer 27

Barrier thick in first trimester
- syncytiotrophoblast, cytotrophoblast and vascular mesoderm
Much thinner by full term
- cytotrophoblast layer lost

Question 28

Placental barrier

Answer 28

By 4th month has
- maternal portion - decidua basalis
- foetal portion - chorion frondosum - formed as more villi develop on embryonic pole
During 4/5th month decidua form decidual septa
- project into intervillous space but do not join chorionic plate
- core of maternal tissue by covered in syncytial cells
- syncytial layer separated maternal blood in intervillous lakes from foetal tissue of villi
- septa divide placenta in to compartments = cotyledons

Question 29

Full term placenta

Answer 29

Discoid shape with diameter of 15-25cm, 3cm thick and weighs 500-600g
Torn from uterine wall 30mins post birth of baby
Maternal side has 15-20 bulging areas which are cotyledons - covered by thin layer of decidua basalis
- intervillous lakes contain approx 150ml of maternal blood - renewed 3-4 times per imn
Foetal surface covered by chorionic plate
- chorionic vessels converge toward umbilical cord
- covered by later of amnion
Umbilical cord attaches to middle of placenta and perpendicular

Question 30

Placenta at end of pregnancy

Answer 30

Aim to reduce exchange between maternal and foetal circulations

• increase fibrous tissue in core of villus
• thickening of foetal capillary basement membranes
• obliterative changes in small capillaries of villi
• deposition of fibrinoid on surface of villi in junctional zone and in chorionic plate