9 Flashcards
How does gas exchange occur at the placenta?
- diffusion barrier: shall, and decreases as pregnancy proceeds
- fetal blood has lower partial pressure of O2 to allow a gradient transfer from mother to fetus
- maternal pO2 increases only marginally
How can a pregnant woman develop a physiological respiratory alkalosis?
-progesterone causes physiological hyperventilation
Even though fetal blood has low pO2, what factors increase the O2 content?
- fetal Haemoglobin variant
- fetal haematocrit is increased over that in the adult
What factors promote O2 exchange to the fetus at the placenta?
increased maternal production of 2,3 DPG
- secondary to physiological respiratory alkalosis of pregnancy
- normally the curve is shifted to left, but with 2,3 BPG the curve is shifted back to right to reduce Hb maternal affinity for O2
Double Bohr effect
- Mother: as CO2 goes into intervillous blood, pH decreases causing a decreased maternal affinity for O2
- Fetus: fetus is giving up CO2 due to gradient which increases pH and results in increased affinity for O2
Fetal Hb -HbF is predominant by week 12 -2 alpha and 2 gamma subunits -has greater affinity for O2 because it doesnt bind to 2,3 BPG as effectively as maternal Hb See REPRO fetal phys slide 8-10
Explain the CO2 transfer between mother and fetus
- progesterone-driven hyperventilation
- thus lower pCO2 in maternal blood
- creates a concentration gradient
- double Haldane effect occurs
What is the Haldane effect?
- double effect
- mother: maternal Hb gives up O2, it can accept increasing amounts of CO2
- fetus: fetal Hb gives up more CO2 as O2 is accepted
What is fetal distress?
- fetal response to hypoxia
- bradycardia occurs via vagal stimulation to try and reduce the O2 demand required by heart
- smoking can cause chronic hypoxaemia
- can eventually lead to intrauterine growth restriction
- HbF and increased [Hb]
- priority of flow to brain and heart
- fetal chemoreceptors detecting decreased pO2 or increased pCO2
See REPRO fetal phys slide 20
Explain fetal circulation
- receives oxygenated blood from mother via placenta in umbilical vein
- lungs are non-functional
- bypasses the lungs
- returns to placenta via umbilical arteries
- occurs through shunts
List and describe the shunts in fetal circulation
Ductus venosus
- only small amount of blood enters liver because liver is massive and could engulf entire fetal circulation
- ductus venosus allows blood from umbilical vein to shunt directly to IVC
- shunting blood away from liver maintains high level of O2 in the circulation that goes to brain and rest of body
Foramen ovale
- allows most of blood to go from RA to LA
- occurs due to the higher pressure in RA than LA
- at birth pressure reverses and ovale closes
- only small amount of deoxygenated blood returns to LA which allows O2 saturation to be high
Ductus arteriosus
- small amount of blood enters right ventricle due to the CRISTA DIVIDENS
- ensures that RV doesnt atrophy through disuse and allows small amount of blood to travel to lungs to promote its development
- connects pulmonary trunk to aorta
See REPRO fetal phys slide 14-19
What are the hormones necessary for fetal growth?
- insulin
- IGF1 and IGF2
- IGF2 nutrient independent, dominant in first trimester
- IGF2 nutrient DEPENDENT, dominates in T2 and T3
- Leptin: placental production
- plus EGF, TGFa
Explain the effect of nutrition on fetal growth during pregnancy
- Week 0-20 (T1): hyperplasia of cells
- Weeks 20-28 (T2): Mix of hyperplasia and hypertrophy of cells that have developed
- Week 28term (T3): predominantly hypertrophy
- Malnutriton can cause symmetrical or asymmetrical growth
- nutritional hormonal status during fetal life can influence health in later life
See REPRO fetal phys slide 22
What is the difference between symmetrical and asymmetrical growth restriction?
- symmetrical: all parts of the fetus are small
- asymmetrical: “head sparing” aka restriction on abd but it is disproportionately smaller than the head
What is amniotic fluid?
- fluid that fills amniotic sac which surround fetus during pregnancy
- function: protection and contains substances critical for lung development
- volume is proportional to the size of the fetus
How is amniotic fluid produced?
- composed of fetal urine
- production of urine starts around 9 weeks
- up to 800ml/day in T3
How is amniotic fluid recycled?
- inhale fluid by fetus practicing breathing movement
- helps with production of lung especially surfactant
- fetus can also swallow fluid so it goes to GI tract
- eventually becomes meconium
See REPRO fetal phys slide 24
What is the composition of amniotic fluid?
- 98% water
- electrolytes, creatinine, urea, bile pigments, renin, glucose, hormones and fetal cells, lanugo, and vernix caseous
See REPRO fetal phys slide 25
What is lanugo?
-fine hair covering fetus
WHat is vernix caseosa?
-waxy substance that covers fetus
What is meconium?
- debris from amniotic fluid plus intestinal secretions including bile
- it should only be released from fetus after delivery
- if fetus is in distress one clinical sign is meconium staining
What is amniocentesis?
- sampling of amniotic fluid
- allows for collection of fetal cells
- useful diagnostic test (ex. Fetal karyotyping)
- can be used for D
- invasive and does carry risk of miscarriage
Explain physiological jaundice in a fetus
-common in newborns
-during gestation, clearance of bilirubin is handled by placenta
-because delay in newborn’s ability to conjugate and excrete bilirubin
-jaundice should disappear within 24 hours
-otherwise it is pathological
See REPRO fetal phys slide 27
Describe the fetal period
- longest period of development
- 9 weeks to term
- term of physiological maturation of structures and growth
See REPRO fetal g&d slide 3-4
Describe the pattern of weight gain during the fetal period
- main tissue contributing to weight at first is protein because of muscle development
- later on in fetal period, adipose tissue is developed for metabolic purposes and for regulation of heat
See REPRO fetal g&d slide 5-6
Describe the proportion of growth during the fetal period
- at 9 weeks, 50% of length of fetus is head
- as pregnancy proceeds, limbs and trunk elongate so body proportions change
See REPRO fetal g&d slide 7-8
What are the main things for the ante-natal assessment of fetal well-being
Ask mom
- fetal movements (occurs around 20 weeks
- mom is aware and sensitive of movements
Regular measurements of uterine expansion
- Symphysis-fundal height
- non-invasive way to assess growth by measuring length from pubic symphysis to top of fundus of uterus
- at 20 weeks SFH should be roughly at umbilicus
- thereafter, it should fought equal the week of pregnancy (28cm = 28 weeks)
Ultrasound scan (USS)
- safe
- can be used early in pregnancy to calculate age and to rule out ectopic or number of fetuses
- usually carried out at ~20 weeks because structures have developed and big enough to be seen
- in order to assess fetal growth and fetal anomalies
See Wkbook and REPRO fetal g&d slide 9-10
How can you estimate fetal age?
LMP (last menstrual period)
- but prone to inaccuracy
- pregnancy happens roughly two weeks after LMP
Developmental Criteria
- allows accurate estimation of fetal age
- done by comparing USS measurements of fetus to an average value
What is Crown Rump Length (CRL)?
- measurement of dating pregnancy between 7-13 weeks
- due to the linear nature of its progression
- used to estimate EDD
- scan in T1 also used to check location, number, viability
- length of fetus from head to tail
See REPRO fetal g&d slide 12
What is biparietal diameter?
- the distance between the parietal bones of the fetal skull
- used in combination with other measurements to date pregnancies in t2 and T3
What is abd circumference and femur length?
- these measurements are used in combination with biparietal diameter during T2 and T3 (when abd is widest)
- used for dating and growth monitoring
- also useful for anomaly detection
- must look at structures to ensure they’re developing normally such as heart and neural tube
What are 3-D or 4-D USS?
- new wave of obstetric ultrasonography
- doesnt replace standard USS as of yet
See REPRO fetal g&d slide 15
Explain the classification of birth weights (i.e. normal, too big, too small etc.)
- 3500g is NORMAL
- <2500g suggests growth restriction
- > 4500g is macrosomia (maternal diabetes)
- many factors influence birth weight such as if mother is petite, baby will be petite
What are some reasons for low birth weight?
- premature
- could be petite since mom is petite
- may have growth restriction (associated with neonatal morbidity and mortality)
How is the respiratory system developed in the fetus?
- lungs develop relatively late
- lungs develop as an outouching of foregut to create the bronchopulmonary tree during embryonic period
- budding and branching of bronchioles starts in week 8-16 (pseudoglandular stage)
- further branching happens to form respiratory bronchioles in weeks 16-26 (canicular stage)
- alveoli develop from ends of respiratory bronchioles at weeks 26 to term
- only then doe Type 1 and 2 pneumocytes develop in order to produce surfactant
See REPRO fetal g&d (slide 19-23)
What determines fetal “viability”?
- whether pneumocytes are present or not
- otherwise gas exchange cant occur
What happens to the lungs during T2 and T3?
- gas exchange conducted at placenta but lungs must be prepared to assume full burden at birth; they need to practice
- “breathing” movements allows respiratory muscles to work and practice
- conditions of the respiratory musculature
- fluid filled
- crucial for normal lung development
- ensures that baby take in amniotic fluid into lungs
- allows to drive the development for the lungs
What is respiratory distress syndrome?
- often affects infants born pre-maturely
- insufficient surfactant production
- if pre-term delivery is unavoidable or inevitable
- then glucocorticoid treatment (of the mother)
- increases surfactant production in fetus
How is the CVS system developed in the fetus?
- fetus CVS system is arranged to ensure oxygenated blood is collected by umbilical vein at the placenta and is circulated around fetus
- average fetal HR is 110-160 bpm
- HR is acheived around 15 weeks
- must know in order to assess fetal bradycardia as a sign of fetal distress
How does the urinary system develop in the fetus?
- fetal kidney function begins in week 10
- fetal urine majorly contributes to amniotic fluid
- fetal kidney function isnt necessary for survival in utero but without it oligohydramnios can occur
- largely reliant on maternal renal function for excretion
What is oligohydramnios and polyhydroamnios?
Oligohydramnios
- too little amniotic fluid
- may be due to placental insufficiency
- can indicate poor renal function in the fetus
Polyhydramnios
- too much amniotic fluid
- indication of issue in recycling amniotic fluid
- such as problem in swallowing (CNS defect) or congenital abnormalities (eg. Trachea-oesophageal fistula)
How does the nervous system develop in the fetus?
-first to start developing and last fo finish
-so it is most vulnerable to injury
-coordinated voluntary movements require the CORTICOSPINAL tract which begins to form in the 4th month
-only myelination of brain occurs at 9th month
See REPRO fetal g&d (slide 30)
How do the sensory and motor systems develop in the fetus?
- no movement until 8th week
- thereafter a large number of movements occur
- these all happen to “practice” for life after birth (ex. Suckling, breathing)
- baby is “rehearsing” movements in utero so that it is prepared to do it immediately after birth
See REPRO fetal g&d slide 31
What is “quickening” in regards to the fetus?
- maternal awareness of fetal movements from 17 weeks onwards
- low cost, simple method of ante-part I’m fetal surveillance
- reveal those fetuses requiring follow-up
Describe the development of the lungs and brain in the fetus
See REPRO fetal g&d slide 33
Why might the symphysis-fundal height not correlate to the week of gestation?
-due to polyhydramnios or oligohydramnios
