Objective To prospectively determine the nature of adnexal masses diagnosed during pregnancy, and investigate whether ultrasound was a reliable means of assessing these. Design A single centre prospective observational cohort study. Setting A large tertiary referral London Hospital. Population Pregnant women with an adnexal mass detected at or prior to the 12-week routine antenatal ultrasound. Methods A detailed ultrasound by a level II ultrasound practitioner at the time of detection; at 12 weeks; 20 weeks and 6 weeks postpartum. Main outcome measures Subjective impression of mass; International Ovarian Tumour Analysis simple rules classification; resolution and intervention rate; incidence of complications related to the mass. Results The incidence was 1%. 274 participants were included. Subjective impression was: simple 75.9%; dermoid 29.1%; endometrioma 6.6%; haemorrhagic 3.3%; para-ovarian 2.6%; torted simple 0.7%; decidualized endometrioma 0.4%; fibroma 0.4%; theca luteal 0.4% and borderline ovarian tumour: 0.7%. There was a significant reduction in the volume at each scan (P <0.0001). 74.2% of masses resolved spontaneously. Surgery was performed in 14/274: 2 antenatally, 6 at Caesarean Section and 6 post-partum. In 5/247 (2%) there was complication due to the mass. Using IOTA simple rules, 272/274 (99.3%) (p<0.0001) were classifiable. Only 1/274 (0.4%) had malignant features as per IOTA (p=0.05). As there was no confirmed malignancy, the accuracy of IOTA simple rules could not be calculated. Conclusion Adnexal masses in pregnancy are uncommon and the majority spontaneously resolve. Malignancy is rare, as are complications. In the absence of concerns regarding malignancy or cyst accident, there is no need for additional monitoring of these masses during pregnancy.