All women will then have a routine viability ultrasound scan at between 6 and 7 weeks gestation as per the fertility unit's standard practice. This viability scan was performed by an appropriately trained doctor or nurse following standard operating procedures and using the same ultrasound equipment as the index test. This viability scan plus any subsequent clinical follow-up required i.e. if the diagnosis was not certain following the viability scan alone, will constituted the reference standard. Clinical follow-up will consist of a repeat transvaginal ultrasound (TVS) 7-10 days after the initial viability scan in cases of pregnancies of uncertain viability (defined as the presence of an intrauterine gestation sac of less than 25mm mean diameter with no obvious yolk sac or fetal pole or an intrauterine gestation sac containing a fetal pole of less than 7mm with no obvious fetal heart pulsations) and in cases of pregnancies of uncertain location (defined as no evidence of an intra- or extra-uterine pregnancy or retained products of conception on transvaginal ultrasound scan in the presence of a positive urinary pregnancy test), referral to a local EPAU for monitoring of serial serum human chorionic gonadotropin (β-hCG) levels and subsequent ultrasonography and possibly surgery where indicated according to departmental protocols until a definitive diagnosis can be made. Interpretation of the reference standard will be performed by an experienced gynaecologist without knowledge of the findings from the index test. Any uncertainty regarding the final diagnosis will be dealt with by seeking the opinion of two other senior gynaecologists and gaining a consensus opinion.