Cardiovascular and Haemodynamic Changes in Pregnancy
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Direct answer
Cardiovascular and haemodynamic changes in pregnancy raise cardiac output by roughly 30 to 50 per cent, driven mainly by plasma volume expansion and a faster heart rate. Systemic vascular resistance falls, so blood pressure dips in the second trimester and returns to pre-pregnancy values by term. These shifts explain the soft flow murmurs, oedema and supine hypotension that examiners expect candidates to recognise.
What you must remember
- Cardiac output: rises by roughly 30 to 50 per cent and reaches its plateau in the second trimester.
- Plasma volume: expands by about 40 to 50 per cent, more than red cell mass, which rises by about 20 to 30 per cent.
- Heart rate: rises by about 10 to 15 beats per minute by term.
- Systemic vascular resistance: falls through progesterone-mediated vasodilatation, reaching its lowest point in mid-pregnancy.
- Blood pressure: diastolic pressure falls most in the second trimester and recovers to pre-pregnancy levels by term.
- Labour: each uterine contraction shifts roughly 300 to 500 mL of blood into the circulation, an autotransfusion effect.
- Supine hypotensive syndrome: the enlarged uterus compresses the inferior vena cava and aorta when the woman lies flat, and left lateral tilt relieves it.
- Puerperium: cardiac output peaks immediately after delivery and falls back over the following weeks.
How the circulation adapts trimester by trimester
The cardiovascular system adapts in a predictable sequence, and exam questions usually ask for the expected finding at each point. Learn the direction and timing of each change rather than memorising isolated numbers.
- First trimester: vasodilatation driven by progesterone lowers systemic resistance early, while cardiac output starts to climb.
- Second trimester: plasma volume expands steeply, cardiac output reaches its plateau, and diastolic pressure falls to its lowest level. A reading that looks low at this stage is often normal.
- Third trimester: blood volume and cardiac output stay high, and the heart rate remains raised by about 10 to 15 beats per minute. Positioning becomes important because of aortocaval compression.
- Labour: each contraction autotransfuses roughly 300 to 500 mL of blood, while pain and anxiety add further rises in heart rate and blood pressure.
- Puerperium: the uterus involutes and extravascular fluid returns to the circulation, causing diuresis. Haemodynamics move back towards pre-pregnancy values over the following weeks.
Where the exam tries to catch you
Examiners choose physiological values that look abnormal, so the traps depend on knowing the normal range.
- Trap: labelling the second-trimester blood pressure fall as hypotension needing treatment Correct: the diastolic fall is physiological, but a reading of 140/90 mmHg or more is not and needs assessment for hypertensive disorders.
- Trap: reading a soft systolic murmur as valvular disease Correct: a soft flow murmur is common because cardiac output is raised, so only a murmur with other features needs evaluation.
- Trap: leaving a pregnant woman supine in an emergency Correct: tilt her to the left to relieve aortocaval compression, since the supine position can drop cardiac output sharply.
Frequently asked questions
What is the normal rise in cardiac output in pregnancy?
Cardiac output in pregnancy rises by roughly 30 to 50 per cent over the non-pregnant value, mainly through a larger stroke volume and a faster heart rate. The rise is established by the second trimester and persists until term.
Why does blood pressure fall in the second trimester of pregnancy?
Blood pressure falls in the second trimester of pregnancy because systemic vascular resistance drops through progesterone-mediated vasodilatation. Diastolic pressure is lowest at this stage and returns to pre-pregnancy values by term.
What is supine hypotensive syndrome in pregnancy?
Supine hypotensive syndrome is a fall in maternal blood pressure and cardiac output that occurs when a pregnant woman lies flat, because the enlarged uterus compresses the inferior vena cava and aorta. It is relieved by left lateral tilt.
How much blood enters the circulation with each uterine contraction?
Each uterine contraction in labour shifts roughly 300 to 500 mL of blood from the uterus into the maternal circulation, an effect called autotransfusion. This temporary boost helps explain why cardiac output is so high during labour.
Why is cardiac output highest immediately after delivery?
Cardiac output peaks immediately after delivery because autotransfusion from the contracting uterus adds blood to the circulation and the caval compression of pregnancy is removed. Output then falls back toward pre-pregnancy values over the following weeks.