L18 Physiology of pregnancy Flashcards
What is there an increase in physiological demand for by the foetus during pregnancy
- Nutrients(eg. O2, amino acids, glucose)
- Amniotic fluid production
- Removal of foetal waste products(eg CO2, nitrogen compounds)
Requires increased:
- Nutrient content (gastro intestinal)
- Oxygen content (pulmonary and cardiovascular)
How much does plasma volume increase by during volume homeostasis
- A rapid increase in plasma volume by 40%
- Plasma colloid osmotic(oncotic) pressure falls –> causes a shift of fluid into extra cellular space, increased hydration of connective tissue and oedema(lower limbs, hands and face)
Mechanism of increased plasma volume
- Slight decrease atrial natriuretic peptide (ANP)
- Decreased thirst threshold (increased fluid intake)
- Re-setting osmostat
- Increased plasma volume
Changes in haemoglobin levels during pregnancy
- Red cell mass increased by 25%
- Plasma volume increased by 40%
- Dilutional anaemia
- Iron is required for the increased red cell mass(fall in ferritin levels, increased iron absorption from gut)
Is Fe supplementation required for twins
- No need for routine Fe supplementation except for twins
Changes in uterine blood flow
- Uterine blood flow increases 3.5 fold from 95 to 342 ml/min
Features of haemostasis
- Hypercoagulable state - increase plasma fibrinogen(increased ESR), platelets, factor VIII and von willebrand factor
- Marked effect at delivery - 500ml/min blood loss at placental separation, myometrial contraction(10% of all fibrinogen used up)
- Evolutionary balance between thrombosis and haemorrhage
Changes in white blood cell levels during pregnancy
- Concentration does not fall during pregnancy
- Total WBC increases in pregnancy
- Increase in neutrophils(reduced apoptosis)
- Marked increased around delivery
Implications of increased blood volume on the CVS
Increased blood volume has implications on:
- Cardiac output
- Peripheral resistance
- Blood pressure
Changes in the heart during pregnancy
- Heart enlarges by 12% (increased venous return)
- Innocent systolic murmurs are common(-90%)
- Diastolic murmurs(-20%) - require investigation ro rule out other pathologies
- Uterus pushing up against the diaphragm can cause the maternal heart to shift up in the chest cavity
Other pathologies that may cause diastolic murmurs
- May be innocent - reflecting increased flow across atrioventricular valves
- Will require further investigation to rule out cardiopathies - but be aware… change in cardiac axis/position result in changes on ECG and x-ray
Changes in peripheral resistance during pregnancy
- Peripheral vasodilatation (effect of progesterone)
- Peripheral resistance decreases by 35%
What is the decrease in peripheral resistance partly compensated by
- Decrease in resistance is partly compensated by an increase in cardiac output which results in a small change in BP
What is increased pulmonary blood flow during pregnancy matched by
- Increase in tidal flow
- Decrease in maternal pCO2 and increase in maternal pO2
- Increased availability of O2 to tissues and aids passive diffusion at the placenta i.e higher concentration gradient
Effects of cardiovascular and respiratory changes
- High blood flow maximises pO2 on maternal side of the placenta
- Foetal haemoglobin(HbF) has a higher affinity for O2 compared with maternal adult Hb(HbA)
- Increased cardiac output may increase flow in skin aiding heat loss (high metabolic state)
Changes in the renal system during pregnancy
- Kidney increases 1cm in size during pregnancy
- GFR and effective renal plasma flow increase 50+%
- But tubular reabsorption capacity is unchanged –> leads to a decrease in glucose reabsorption thus glycosuria is common
Changes in plasma levels of creatinine and urea
- Plasma levels of creatinine and urea decrease in pregnancy
- All the increments are present by the second trimester
Change in GFR during the third trimester
- Reduction GFR of 15% during the third trimester
Changes in renal pelvis and ureters
- Dilatation of renal pelvis and ureters (progesterone) - increased urinary tract infections in pregnancy
Changes in GI system
- Gastro-oesophageal reflux up to 70%
- Slowing of gut motility and constipation (progesterone effect)
Why is there an increase in gastro-oesophageal reflux during pregnancy
- Due to increase in abdominal pressure, reduced pyloric sphincter with back wash of bile secondary to hormonal changes
- Simple measures - avoidance of fat and alcohol
- Upright posture and antacids
Glucose metabolism - first trimester
- Increased sensitivity to insulin thus mothers increase glycogen synthesis and fat deposition
Glucose metabolism - second trimester
- Insulin resistance
- Cortisol, progesterone, HPL, and oestrogen are all insulin antagonists
- Thus glucose levels may rise and there is an increase in fatty acids(another source of energy for the fetus)
Why is folate important
- DNA synthesis, repair and regulation
- RBC development
- Plasma folate represents current nutritional status, but
- Significant tissue stores(eg liver) - RBC folate is a good biomarker(no change in pregnancy) –> dietary deficiency can take months to become significant
- No need for folate supplementation but prevents neural tube defects thus routinely given preconception to 3 months
Why is folate important for DNA synthesis, repair and regulation
- Important in rapid cell division (embryos)
- Deficiency in pregnancy associated with neural tube defects (NTDs)
What type of anaemia can result from folate deficiency
- Macrocytic anaemia
Changes in thyroid function - pregnancy
- Increased iodine absorption
- Increased serum T3 and T4 levels
- Increase in thyroid binding globulin(oestrogen)
- As only unbound T3 and T4 is active, levels of free T3 and T4 remain the same or fall slightly
- In general, thyroid function remains unchanged
- if hypothyroid may need to increase dose due to increased TBG levels
Protein hormones produced by the placenta
- hCG
- hPL
- hPG
- CRH
Steroids produced by the placenta
- Progesterone
- Oestrogen(oestriol)
When is hCG first detectable
- First detectable 8-9 days after ovulation and peaks at 8-10 weeks
Which subunit from hCG is used as the pregnancy test
- Beta subunit is used as the pregnancy test
Changes in hCG levels during pregnancy
- Doubles every 48-72 hours
What layer of the embryo produces hCG
- Produced by the trophoblast
- Produced in large quantities by hydatidiform molar pregnancy and choriocarcinoma
When are hCG levels usually lower during a pregnancy
- Usually significantly lower in ectopic pregnancy and risk of miscarriages
What is the alpha subunit of hCG similar to
- Alpha subunit very similar to LH, FSH, TSH
- Has LH type properties but longer half life (24 h)
effects of hCG
- Maintains corpus luteum secretion of progesteron and oestrogen
- Decreases as the placental production of progesterone increases
- Later in pregnancy may have a role in maternal oestrogen secretion and modulation of the maternal immune response
What is hPL similar to
- Similar structure to prolactin and growth hormone
Link between placenta size and hPL levels
The bigger the placenta, the more hPL
Half life of hPL
Half life - 30 mins
Effects of hPL
Alters maternal carbohydrate and lipid metabolism to provide for foetal requirements
- Mobilising maternal free fatty acids
- Inhibits maternal peripheral uptake of glucose
- Increases insulin release from pancreas
Aim is a steady state of glucose for the fetus
Effects of hPG
- Placental growth hormone secreted by the placenta responsible for regulating fetal growth
- Induces maternal insulin resistance
- No evidence of that maternal GH or fetal GH required for fetal growth
Effects of placental CRH
- Stimulates production maternal: ACTH and cortisol
- Increased cortisol believed to be detrimental to the foetus
- Increased cortisol can result in increased maternal glucose
Why is increased cortisol levels believed to be detrimental to the foetus
- High levels early linked to slower rate of cognitive development post-partum
- High levels late linked to accelerated cognitive development post-partum
Effects of progesterone
- Maintains uterine quiescence by decreasing uterine electrical activity
- Immune suppressor (HLA)
- Lobulo-alveolar development in breasts
- Substrate for fetal adrenal corticoid synthesis eg cortisol
Effects of oestrogen
- Growth of the uterus, cervical changes
- Development of ductal system of breasts
- Stimulation of prolactin synthesis
- Stimulation of corticol binding globulin (CBG), sex hormone binding globulin(SHBG), thyroxin binding globulin (TBG)
- Both maternal and foetal dehydroepiandrosterone(DHEA-S) is converted to oestriol(asomatase)
What is most of oestrogen produced as
- 90% as oestriol(to modulating uteroplacental blood flow)
