Cardiac output (CO) is the volume of blood ejected from the heart and is a product of stroke volume (SV) vs heart rate (HR). It is closely related to Blood Pressure (BP) by the formula CO = Mean Arterial Pressure (MAP) / Systemic Vascular Resistance (SVR). Cardiac output monitoring is well established and validated for guiding fluid administration and resuscitation in intensive care and perioperatively in the non-pregnant population. Available CO monitoring methods may risk maternal health or fetal wellbeing or are unvalidated in the pregnant population. Invasive and semi-invasive methods of measuring CO such as the pulmonary artery catheter, the 'gold standard' require invasive arterial or central access negating their use in all but a select group of labouring women and have potentially serious risks attached to their usage. Echocardiography is a well-established and validated technique requiring no invasive access but the requirement for an experienced operator limits routine usage. It uses an external ultrasound probe to scan the heart.