Sinus tachycardia (normal heart rhythm but fast heart rate) is common and usually related to an obvious cause (e.g. exercise such a running or cycling) and doesn't cause any untoward symptoms. In rare cases however, sinus tachycardia can occur without an obvious cause (i.e. it is inappropriate to the situation) and can cause distressing symptoms such as palpitations, dizziness and breathlessness at rest or with minimal exertion. This syndrome of inappropriate sinus tachycardia is poorly understood and can be difficult to manage; especially when the affected individual is pregnant. In order to understand the condition better and improve care pathways we aim to measure various features of the cardiovascular system in pregnant women with and without IST.