Repro: Pregnancy and Adaptation Flashcards
What is the week of 2’s?
Two distinct cell layers
Outer cell mass
- Syncytiotrophoblast
- Cytotrophoblast
Inner cell mass becomes the bilaminar disk
- Epiblast
- Hypoblast
What are 2 cavities in the embryo?
- Amniotic cavity
- Yolk sac
How is the embryo suspended in the chorionic cavity?
-Connecting Stalk
What is the fate of embryonic spaces?
- The yolk sac disappears
- The amniotic sac enlarges
- The chorionic sac is occupied by the expanding amniotic sac
What does implantation achieve?
Establishes the basic unit of exchange
-Primary villi: early finger-like projections of trophoblast
-Secondary villi: invasion of mesenchyme into core
-Tertiary villi: invasion of mesenchyme core by fetal vessels
Anchor the placenta
Establish maternal blood flow within the placenta
How does membrane change through pregnancy?
- Implantation is interstitial (the uterine epithelium is breached and the conceptus implants within the stroma)
- The placental membrane becomes progressively thinner as the needs of the fetus increase
- In the human one layer of trophoblast ultimately separates maternal blood from fetal capillary wall (but the two circulations never mix)
What is a chorionic villus?
The placenta is a specialisation of the chronic villus. The chorion fondusum is the outer layer and has fingerlike projections. This allows finger-like projections from trophoblast that have vessels to allow for good exchange.
What are the possible implantation defects?
- Impantation in the wrong place (Ectopic pregnancy, Placenta praevia)
- Incomplete Invasion (Placental Insufficiency, Pre-Eclampsia)
Why is ectopic pregnancy bad?
- No decidua therefore no control
- The conceptus can invade into tissues
How does the chorionic villus change from first semester to third semester?
First trimester - Thicker barrier
Third Trimester - Thinner barrier
What is the blood vessels to the umbilicus?
- Two umbilical arteries to transport deoxygenated blood from foetus to placenta
- Umbilical vein to transport Oxygenated blood from placenta to fetus
Which hormones are produced by the placenta?
Protein
- Human chorionic gonadotrophin
- Human chorionic somatomammotrophin
- Human chorionic thyrotrophin
- Human chorionic corticotrophin
Steroid
- Progesterone
- Oestrogen
What is hCG?
- Hormone produced in the first 2 months of pregnancy
- Supports secretory function of corpus luteum
- Produced by syncytiotrophoblast therefore pregnancy specific
- Excreted in eternal urine and therefore used as a basis for pregnancy testing
How do placental hormones influence maternal metabolism?
- Progesterone increases appetite in order to lay down fat stores which will be called upon later in pregnancy
- hCS/hPL increases glucose availability to fetus. Causes insulin resistance in maternal tissue so other stores are used by mother so the foetus gets glucose
What are transport functions of the placenta?
- Simple diffusion (water, electrolytes, urea/uric acid, gases)
- Facilitated diffusion (applies to glucose transport)
How does gas exchange occur in the placenta?
- Simple diffusion
- Diffusion barrier is small and decreases as pregnancy proceeds
- Flow limited, not diffusion limited. Low stores of fetal oxygen so needs adequate low
- Gradient of partial pressure is required so fetal pO2 must be lower than maternal pO2 which increases marginally.
Describe active transport in the placenta
Specific transporters expressed by syncytiotrophoblast for:
- Amino acids
- Iron
- Vitamins
How does transfer of passive immunity occur?
- Fetal immune system is immature
- Receptor mediated process maturing as pregnancy progresses
- Immunoglobulin class-specific
- IgG only. Concentration in fetal plasma exceed this in maternal circulation
Why is the placenta not a true barrier?
- Teratogens can access the fetus via the placenta
- Unintentional outcomes from physiological process. Haemolytic disease of the newborn secondary to Rhesus incompatibility of mother and fetus. Can lead to destruction of metal erythrocytes. It can be prophylactically treated
What are examples of harmful substances and the placenta?
- Thalidomide (Limb defects)
- Alcohol (FAS and ARND)
- Therapeutic drugs (anti-epileptic drugs, warfarin, ACE inhibitors)
- Drugs of abuse (dependancy in the fetus and newborn)
- Maternal smoking
What are the periods of susceptibility to teratogenesis?
Pre embryonic
-Lethal effects
Embryonic
- More sensitive
- Narrow windows for some systems
Fetal
+/- sensitive
After embryonic period, risk of structural defects very low
-Except CNS as development occurs throughout
What are the structures(vessels) of the maternal-fetal exchange that happens at the placenta?
- Fetal circulation
- Umbilical arteries (deoxygenated blood)
- Umbilical vein (oxygenated blood)
- Fetal capillaries within chorionic villi (increase surface area)
- Uterine arteries
- Uterine veins (maternal blood lakes in the intervillous spaces)
What factors increase fetal O2 content?
- Fetal haemaglobin variant
- Fetal haematocrit is increased over that in the adult
- Increased maternal production of 2,3 DPG secondary to physiological respiratory alkalosis of pregnancy
- Double Bohr effect
What is the fetal haemoglobin variant?
- 2 alpha subunits plus 2 gamma subunits
- Greater affinity fo oxygen because it doesn’t bind 2,3-DPG as effectively as HbA
Describe the double Bohr effect in materno-fetal exchange.
- As CO2 passes into intervillous blood, pH decreases
- Bohr effect
- Decreasing affinity of maternal Hb for O2
- At the same time, as CO2 is lost, pH rises in the fetus
- Bohr effect
- Increasing affinity fo Hb for O2
How does the concentration gradient for CO2 transfer form?
- Maternal physiological adaptation to pregnancy
- Progesterone-driven hyperventilation
- Hence lower pCO2 in maternal blood
What is the double Haldane effect?
- As Hb gives up O2, it can accept increasing amounts of CO2
- Fetus gives up CO2 as O2 is accepted
- No alteration in local pCO2
Describe the fetal circulation.
- Receives oxygenated blood from mother via placenta in umbilical vein
- Lungs are non functional so they are bypassed
- Returns to the placenta via umbilical arteries
What are the 3 shunts found in the fetal circulation?
- Ductus Venosus
- Foramen ovale (Right atrium to left atrium to by pass lungs)
- Ductus arteriosus (pulmonary trunk to aorta)
What is the purpose of the ductus venosus?
- Connects umbilical vein carrying oxygenated blood to the IVC
- Blood enters the right atrium so blood is shunted around the liver to maintain most of the saturation
What is the purpose of the foramen ovale?
- Right atrial pressure is greater than in the left atrium in the foetus
- Forces leaves the foramen ovale part and blood flow into left atrium
- Free border of septum secundum forms a crest
- Creates two streams of blood flow
- Majority of blood flow to Left atrium and the minor flows to Right ventricle
Why is the ductus arteriosus needed?
- Shunts blood from Right ventricle and Pulmonary trunk to Aorta
- Minimuses drop in O2 saturation
What is the fetal response to hypoxia?
- HbF and Hb is increased
- Redistribution of flow to protect supply to heart and brain
- Fetal heart rate slows in response to hypoxia reduce O2 demand
What is the effect of chronic hypoxaemia?
- Growth restriction
- Behavioural changes
How is hypoxia detected in the foetus?
- Fetal chemoreceptors detect decreased pO2 or increased pCO2
- Vagal stimulation then leads to bradycardia
Which hormones are necessary for foetal growth?
- Insulin
- IGF1 and IGF2
- Leptin
What are the effects of nutrition on fetal growth during pregnancy?
- Can cause symmetrical or asymmetrical growth restriction
- Can influence health in later life
What is the dominant cellular growth mechanism in the first, second, and third trimester?
First trimester - Hyperplasia
Second trimester - Hyperplasia and Hypertrophy
Third trimester - Hypertrophy
What is the purpose for amniotic fluid?
- Amniotic sac encloses embryo/fetus in amniotic fluid
- Protection
- Contributes to development of lungs
How is the amniotic fluid produced and recycled?
- Fetal urinary tract produced urine by 9 weeks
- Fetal lungs
- Fetal GI tract (Swallowed and absorbs water and electrolytes. Debris and intestinal secretions accumulates in Gut to form meconium)
- Placenta and fetal membranes
What is the composition of amniotic fluid?
- 98% water
- Plus electrolytes, cretinne, urea, bile pigment, renin, glucose, hormones and fetal cells, lung and Venice caseosa
What is amniocentesis?
- Sampling of amniotic fluid
- Allows for collection of fetal cels
- Useful diagnostic test
How is the bilirubin metabolised in the foetus and why?
- During gestation, clearance of fetal bilirubin id handle efficiency by the placenta
- Foetus cannot conjugate bilirubin
- Immaturity of liver and intestinal processes for metabolism, conjugation and excretion
- Physiological jaundice common
What is the pattern of growth during development?
- Embryonic period is characterised by intense activity but absolute growth is very small
- Growth and weight gain accelerate in fetal period
Embryo - Intense morphogenesis and differentiation; little weight gain; placental growth most significant
Early fetus - Protein deposition
Late fetus - Adipose deposition
How do body proportions change during detail period?
- At 9 weeks the head is approx half crown-rump length
- Thereafter, body length and lower limb growth accelerates
What is an obstetric ultrasound scan?
- Safe method that can be used early in pregnancy to calculate age. This can rule out ectopic and number of foetuses.
- Routinely carried out at 20 weeks to assess fetal growth and fetal anomalies
How is crown-rump length used to date the pregnancy?
- Measured between 7 and 13 weeks to date the pregnancy and estimate delivery date
- Scan at T1 also used to check location, number, viability
How can the fetal age be estimated?
- Last menstrual period (prone to inaccuracy)
- Development criteria
How is biparietal diameter used to date the pregnancy?
- Distance between the parietal bones of the fetal skull
- Used in combination with other measurements to date pregnancies in T2 and T3
How is abdominal circumference and femur length to date the pregnancy?
- AC and FL used in combination with biparietal diameter for dating an growth monitoring
- Also useful for anomaly detection
How are birth weight classified?
3500g - Average
<2500g - suspects growth restriction
>4500g - macrosomia (maternal diabetes)
Why can babies have a low birth weight?
- Premature
- Constitutionally small
- Suffered growth restriction
Describe an overview of the development of the Respiratory system?
- Lungs develop relatively late
- Embryonic development creates only the broncho pulmonary tree
- Functional specialisation occurs in the fetal period
- Stage of development has major implication for pre-term survival. Terminal sac stage is crucial by 24 weeks
Outline the stages of lung development.
Pseudoglandular Stage - Duct system (bronchioles) forms with the bronchopulmonary segment created during the embryonic period. (Wks 8-16)
Canalicular stage - Formation of respiratory bronchioles budding from bronchiole formed previously (Wks 16-26)
Terminal sac stage - Terminal sacs begin to bud from respiratory bronchioles. Differentiation of Type 1 and Type 2 pneumocytes. Surfactant. (Wks 26)
How are lungs prepared to assume full burden at birth at T2 and T3?
- Breathing movement to condition the respiratory musculature
- Fluid filled (amniotic) which is crucial for normal lung development
What is respiratory distress syndrome?
- Insufficient surfactant production
- Often affects infant born prematurely so if delivery pre-term in unavoidable then glucocorticoid treatment to increase surfactant production
Describe an overview of the urinary system.
- Fetal kidney function begins in week 10
- Fetal urine is a major contributor to amniotic fluid volume
- Fetal kidney fucntion is not necessary for survival but without it there is oligohydramnios
What is the importance of amniotic fluid volume?
Oligohydramnios
- Too little
- Can cause placental insufficiency and fetal renal impairment
Polyhydramnios
- Too much
- Fetal abnormality due to inability to swallow so recycling process is implicated
Describe the overview of nervous system.
- First to being development and last to finish
- Corticospinal tract required for coordinated voluntary movement being to form in the 4th month
- Myelination of brain only begins in 9th month
When does movement develop in the foetus?
- No movement until after 8th week
- After large repertoire of movement develop to practise for post-natal life (suckling, breathing)
What is quickening?
- Maternal awareness of fetal movement from 17 weeks onwards
- Low cost, simple method of antepartum fetal surveillance
- Reveal those fetuses requiring follow-up
