Objective To assess the prevalence of endometriosis using pelvic ultrasound examination in women attending for early pregnancy care Design Prospective observational study. Setting A dedicated early pregnancy unit. Population We included 1341 consecutive women who attended for an early pregnancy assessment and had transvaginal ultrasound scans performed by a single clinician. Methods In addition to the presence of endometriosis on ultrasound scan, we collected data on patient demographics and concurrent gynaecological conditions. Data analysis was performed using logistic regression and multivariate analysis. Main outcome measures The prevalence of endometriosis in addition to possible associations with demographic and clinical variables. Results The prevalence of endometriosis in women attending the early pregnancy unit was 4.9% (95% CI 3.8 – 6.2); In 33/66 (50%, 95% CI 37.9 – 62.1) women with endometriosis, this was a new diagnosis made for the first time during their early pregnancy scan. The presence of endometriosis was strongly associated with congenital uterine anomalies (p < 0.001; OR 5.69, 95% CI 2.17 – 14.9) and uterine fibroids (p = 0.004; OR 2.37, 95% CI 1.31, 4.28). Conclusions Endometriosis is present in nearly 5% of women attending for early pregnancy assessment. In half of the women with endometriosis, the diagnosis was made for the first time during pregnancy. We propose that ultrasound may be a useful tool for the detection of endometriosis and to identify pregnant women who may benefit from specialist antenatal care. Funding No funding was obtained for this work. Keywords Endometriosis, Prevalence, Pregnancy, Ultrasonography