Changes in Maternal Physiology During Pregnancy

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There are a number of changes in maternal physiology during pregnancy that are summarised in this article.

Weight Gain

  • Overall gain = ~12.5 kg
    • Foetus = 3.5 kg
    • Placenta = 0.65 kg
    • Uterus = 1 kg
    • Breasts = 0.5 kg
    • Blood and fluid retention = 2 kg
    • Maternal fat = 4 kg

Cardiovascular Changes

Blood volume

  • Total blood volume increases by 1.5L (40%)
  • Total plasma volume increases by 1.25L (45%)
  • Total red blood cell mass increases by between 240ml (18% - without iron supplements) and 400ml (30% - with iron supplements)
    • Haemocrit falls (40% to 31%)
    • Haemoglobulin concentration falls from 13.5% to 11-12%
    • Results in dilutional anaemia of physiological anaemia of pregnancy
  • White blood cell concentration increases from 7000/ul to 10-11,000/ul
  • Platelet turnover increases
  • Total plasma proteins increase, but concentration decreases
  • Globulin increases, albunin:globulin ratio falls

Cardiac Output

  • Cardiac output increases by approximately 1.5L/min
    • Stoke volume increases from 64ml to 71ml
    • Heart rate increases from 70bpm to 85bpm
  • Total peripheral resistance falls
  • Blood pressure falls during first trimester, but returns to normal by term

Cardiac distribution

Increased flow to:

  • uterus (from 50ml/min to 700ml/min, 90% to placenta)
  • breasts
  • kidneys (30%)
  • skin

Venous pressure in lower body increased due to:

  • mechanical pressure of uterus
  • haemodynamic effect of increased flow to uterus
  • veins more distensible (valves weakened) due to progesterone

Renal Changes

  • Increased blood flow (40%)
  • Increased plasma flow (45%)
  • Increased glomerular filtration rate (50% - due to increased plasma flow and reduction in colloid osmotic pressure)
  • Reduced levels of creatine and urea in plasma
  • Failure to reabsorb all filtered glucose - glycosuria
  • Amino acid excretion increased
  • Increased sodium retention (largely due to activation of renal angiotensin system)
  • Increased water retention
  • In bladder frequency (both day and night)
    • Due to increased GFR, compression by uterus and hyperaemia
    • severe by week 6 then subsides until late pregnancy

Respiratory Changes

  • Minute ventilation increases
  • Respiratory rate constant
  • Tidal volume increases - promoted by softening of thoracic ligaments and rising of diaphragm by 4 cm
  • Functional residual volume decreases (reduces dilution of breath)

Gastrointestinal Changes

  • Progesterone induces cravings
  • Increased absorption of calcium and iron
  • lower and upper intestines raised
  • decreased motility of GIT (increased absorption, but leads to constipation)
  • reduced tone of lower oesophageal sphincter (due to progesterone, leads to heartburn)
  • increased volume and contraction of gull bladder (leads to kidney stones)

Metabolic Changes

Metabolism increases in general. Changes in:

  • Carbohydrates (decrease in glucose levels in first trimester, increase in insulin resistance in third trimester due to human placental lactogen - induces metabolism of fat into glycogen which is diffused over placenta, but increases peripheral insulin sensitivity)
  • Proteins (amino acid concentration falls due to active transport over placenta, require high fibre diet to compensate)
  • Fat (fatty acids become main energy source and stores increase by 4 kg)

See also