Ovarian tumours are swellings affecting the ovary (a tumour is not necessarily a cancer) which may be cystic (i.e. filled with fluid) or solid. Large cysts are at risk of twisting round their blood supply and causing pain - a process called torsion - or of obstructing labour and delivery, while solid ovarian swellings might possibly be cancerous. For these reasons it may be best to remove such swellings once a pregnancy has become established and is not dependent on the function of the corpus luteum - the remains of the follicle from which the egg emerged. Small cysts can be monitored using ultrasound: many disappear during the course of the pregnancy.