The incidence of maternal collapse and severe maternal morbidity is unknown. Recent studies estimate that maternal collapse occurs in between 0.14 and 6 per 1000 births [1, 2]. In hospital, maternal collapse and sudden cardiac arrest are usually related to peripartum events and the outcome depends on effective resuscitation and identification and effective treatment of the underlying cause [3]. Therefore, staff on the delivery suite must be expertly trained in advanced life-support techniques and resuscitation equipment should be readily available.

The maternal body accumulates water in pregnancy. Total body water can increase by up to 8 L. Oestrogens increase plasma renin activity with enhanced renal sodium absorption and water retention (renin–angiotensin–aldosterone system). Plasma osmolality decreases by about 10 milliosmol/kg below non-pregnant levels [1, 2]. These changes promote sufficient placental perfusion as the plasma volume rises steadily throughout the first two trimesters and plateaus at around 32 weeks. Reduced maternal plasma volume is associated with abnormal pregnancy outcome [3].

The normal human heart is able to tolerate recurrent episodes of pregnancy-related volume overload without compromising normal function [4]. Disturbances in the cardiovascular system and the regulation of fluid balance can lead to changes in blood pressure and to hypotension or hypertension. Imbalance in salt and fluid regulation is common and can cause oedema in later stages of pregnancy.