AbstractObjectives: The primary objective of this study was to ascertain whether there is association between low initial serum progesterone, sonographic parameters and clinical outcomes in women presenting with pregnancies of unknown location (PUL), which are found to be ongoing at their follow up scans in the first trimester.Study design: This was a non-interventional retrospective cohort study of 1056 patients spanning a 14-year period, conducted in the Early Pregnancy Unit (EPU) of an inner-city teaching hospital. Patients who had an ongoing singleton first trimester pregnancy after presenting with PUL were identified and categorised as having low progesterone if it was 32 nmol/l or lower. The crown-rump length (CRL), mean gestational sac diameter (MGSD) and gestational sac volume (GSV) were measured when the embryo was first seen, and the pregnancy outcome recorded.Results: Pregnancies with low progesterone tended to have smaller gestational sacs (GS) on follow up scan (p = 0.001) and the sac was smaller than expected for a given CRL (p = 0.000). There was no ultrasound parameter that was characteristic of low progesterone. The observation of a smaller than expected MGSD for a given CRL remained even when only pregnancies with normal outcomes were analysed. Clinical outcome data were available for 854 (80.9 %) women. Overall, 81.4 % (n = 34/43) of pregnancies with low progesterone resulted in livebirth, compared to 91.7 % (n = 744/811) livebirths in pregnancies with higher levels (p = 0.0454).Conclusion: Pregnancies with low progesterone tend to have a smaller GS compared to those with a higher progesterone, and the GSs are smaller than expected for a given CRL. The current study shows that women with low progesterone at the start of pregnancy remain at higher risk of miscarriage, even when the pregnancy is initially found to be viable in the first trimester. These pregnancies also tend to be associated with the sonographic finding of a smaller GS than expected for a given gestational age, regardless of eventual outcome. (C) 2021 Published by Elsevier B.V.