Expansion
Cardiac output and volume rise, resistance and blood pressure fall
Expansion
Increases
- Cardiac output by 30 to 50 per cent, mostly by mid-pregnancy; through both rate and stroke volume
- Plasma volume by about 50 per cent
- Red cell mass by only about 20 to 30 per cent
- Heart rate by 10 to 20 beats per minute
Decreases
- Systemic vascular resistance, largely through progesterone and the low-resistance placental bed
- Blood pressure, reaching a nadir in the second trimester before returning towards baseline
- Colloid oncotic pressure, predisposing to oedema
Because plasma volume rises more than red cell mass, haemoglobin concentration falls: the physiological anaemia of pregnancy.
From about 20 weeks, lying supine allows the uterus to compress the inferior vena cava, causing aortocaval compression with a fall in venous return and supine hypotensive syndrome. This is why women beyond mid-pregnancy are nursed and resuscitated with left lateral tilt.