Cardiovascular changes in pregnancy

• Cardiac output - 40% increase by 12 weeks gestation from 4.5 to  6L/min. Both heart rate (10% increases) and stroke volume are increased with increased myocardial contractility
• Labour is associated with a further rise in cardiac output of ~40%
• Cardiac output falls rapidly after delivery.
• Peripharal resistance falls during pregnancy, secondary to factors such as oestrogen and nitric oxide
• Blood pressure falls to a nadir at ~24 weeks gestation then rises to pre-pregnancy values at term
• Blood pressure increases in labour
• Heart position is elevated by diaphragm and the apex beat moves to fourth left inter-costal space
• ECG shows deviation of electrical axis to the left by 15 degrees; other changes including S-T segment depression and flattening or depression of T waves in III
• Hb concentration decreases during pregnancy but total oxygen carrying capacity of blood is increased (increased total Hb)
• Increased cardiac output exceeds the increase in oxygen consumption hence the arterio-venous oxygen difference is decreased.
• Immediately following delivery, cardiac out-put increases by 10-20% as blood initially within the uterus is returned as the uterus contracts

Respiratory changes in pregnancy

• Progesterone increases the sensitivity of the respiratory centres to CO2 but the respiratory rate is unchanged in pregnancy
• Minute volume - tidal volume X respiratory rate - increased by 50% in early pregnancy. Tidal volume increases with little increase in respiratory rate
• Residual volume - volume of air left in the lungs after the most forceful expiration decreased by 20% as does functional residual capacity and expiratory reserve volume
• Vital capacity and expiratory reserve are unchanged – unchanged
• Physiological dead space increased by dilatation of small bronchioles
• PEFR and FEV1 are unchanged
• There is a fall in arterial PCO2 with little change in PO2. The fall in PCO2 is matched by a fall in plasma bicarbonate (renal compensation - compensated respiratory alkalosis) with no resultant change in pH. pH= 7.44, pCO2=30, bicarbonate=20-25

Haematological changes in pregnancy

• Increased plasma volume in the first trimester (40 - 50%) with a 25-30% increase in red cell mass resulting in haemodilution
• Mean cell volume is increased but mean cell Hb concentration is unchanged
• Platelet count falls at term while the leucocyte count is increased slightly
• Iron demand is increased with increased absorption from the gut. Total serum iron binding capacity is increased with decreased serum iron and serum ferritin. Transferrin concentration increased
• Reticulocyte count may increase in pregnancy

Coagulation changes in pregnancy

• Concentrations of clotting factors VII - X. fibrinogen increased in pregnancy and remain elevated in the puerparium, accounting for the increased risk of thrombosis. This is true for carriers of haemophilia and women with von Willebrand disease
• Anticoagulants - Antithrombin III concentration is unchanged
• Protein C, Alpha-1 antitrypsin and alpha-2 macroglobulin concentrations increase
• Protein S concentrations fall
• Fibrinolysis - increased inhibition of fibrinolysis: plasminogen activator inhibitor concentrations are increased as it is produced by the placenta
• Fibrin degradation products - concentrations rise in the third trimester

 Renal changes in pregnancy

• Increase in kidney size and weight, ureteral dilatation (Right > left), bladder becomes an intra-abdominal organ
• GFR increases 50%, renal plasma flow increases by 75%. Peak GFR reached ~16-24 weeks gestation. GFR falls in late pregnancy
• Creatinine clearance increases to 150-200 ml/min
• Serum urea & creatinine decreases by about 25% to ~ 3.6mM and 60mM respectively
• Decreased uric acid concentration in early pregnancy due to increased clearance. Levels increase in the third trimester
• Marked increase in renin and angiotensin concentrations, but markedly reduced vascular sensitivity to their hypertensive effects
• Aldosterone secretion increased as a consequence of activation of renin-angiotensin pathway - 6-8x non-pregnant. Increases salt and water reabsorption from the renal tubules off-setting the increase in GFR
• Progesterone has a natriuretic effect and stimulates potassium loss - this is balanced by the effects of aldosterone. Overall, there is a small degree of salt and water retention in pregnancy
• Total body water increases by 6-8L. Extracellular fluid volume increased by 3L, about 1.5L of which is plasma
• Increase in glucose excretion as filtered glucose load may exceed renal threshold for absorption
• Increased renal protein excretion - up to 300mg / 24h is normal. Amino acid excretion is increased
• Thus, mild glycosuria (1-10 gm/day) and/or proteinuria (to 300 mg/day) can occur in normal pregnancy
• Urine volume is not changed

GI changes in pregnancy

• Decreased motility, probably due to influence of progesterone
• Reduced gastric acid secretion
• Peptic ulceration is rare during pregnancy and if present pre-pregnancy, may improve
• Relaxation of lower oesophageal sphincter - increased risk of reflux
• Constipation more common - compression of rectum by uterus, increased water absorption caused by increased angiotensin II and reduced smooth muscle activity caused by progesterone
• Gall stones more common - smooth muscle relaxation cause sluggish flow of bile. Liver function and bilirubin concentration unchanged
• Serum albumin concentration falls by 20% but there is a slight increase in total protein concentration