Progesterone reduces systemic vascular resistance by about 20% early in pregnancy. Postural hypotension may result.
Diastolic and systolic blood pressure tend to fall during mid pregnancy and then return to normal by week 36.
Venous return in the inferior vena cava can be compromised in late pregnancy if a woman lies flat on her back. This is relieved by lying in the left lateral position.
Increased circulating angiotensin II encourages water and sodium retention, leading to an increased plasma volume (to 50% by 30 weeks) and predisposing to oedema. This enables increased uterine blood flow to meet growing nutritional and oxygenation needs of the fetus. It also enables blood loss (average 500 ml) at delivery to be met without physiological decompensation.
Advise women not to take up unaccustomed, vigorous exercise in pregnancy as there is a risk of diversion of uterine blood flow to the skeletal muscles.
Blood flow to kidneys, skin and mucosa increases.
Cardiac output increases by 30-50% with 15% increase in heart rate and 25-30% increased stroke volume. Much of this adjustment occurs prior to 12 weeks of gestation and so impaired cardiac function is likely to present problematically in early pregnancy or with the sudden increase in pre-load in the third stage of labour.