Maternal Physiology

Cardiovascular system

• Fall in total peripheral resistance by 6 weeks gestation to a maximum of ~40% at mid-gestation
  • Thus drop in afterload => perceived circulatory underfilling, activating the RAAS to re-expand plasma volume (rises by about 50-60%)
    • Note, however, that total ECF only rises by 16% (so disproportionate rise in PV- osmolality falls by around 10mmol/kg)
• Simultaneous rise in heart rate by 10-15bpm and cardiac output rises by around 35-40% in primiparous women and 50% in subsequent pregnancies
  • Most of this due to increase in stroke volume with enlarged heart chambers and hypertrophy
• Blood pressure tends to fall in the first half of pregnancy (diastolic > systolic) but then begins to climb until term (can rise above patient normal)

Respiratory system

• Tidal volume rises by ~30% in early pregnancy by 40-50% with a fall in expiratory reserve and residual volume due to elevation of the diaphragm and increased sensitivity to CO at the medulla due to progesterone
  • FEV1 and PEFR are NOT affected (even in asthmatic patients); neither is Resp rate (therefore there is a rise in minute ventilation)
  • These changes are markedly increase with exertion
• Progesterone also increases carbonic anhydrase, lowering maternal pCO2 which can compensate for the rise in CO2 from foetal circulation in the third trimester

Urinary/Renal system

• Kidneys increase in size
  • Renal parenchyma (calyces, pelvis and ureters) increases by 70%
    • Can cause stasis and predispose to infection
• Renal blood flow also increases by up to 80% by mid pregnancy then falls slightly (remains >50%)
  • Proportionately greater than cardiac output
• GFR also increases by around 50%
• Tubular reabsorption also increases by a similar amount
• Bladder tone decreases but becomes more irritable

GI System

• Decreased motility in first 2 trimesters – increasing absorption of water and salt (also constipation)
• Heartburn (from increased gastric pressure)
• Albumin and other plasma proteins rise (even with increased Plasma volume)
• Gallbladder empties more slowly and cholestasis is common (can cause itch but rarely jaundice and occasionally gallstones)
• Initially, insulin sensitivity rises (increasing production of stored glycogen and fat).  After mid-pregnancy, insulin sensitivity falls, allowing more glucose to cross the placenta to nurture the growth of the foetus.
  • see also Diabetes in Pregnancy