Why are some ectopic pregnancies characterized as pregnancies of unknown location at the initial transvaginal ultrasound examination?

Objective. To compare the appearance and behavior of ectopic pregnancies (EPs) initially classified as pregnancies of unknown location (PULs) to those visualized on the initial transvaginal ultrasound scan (TVS). Methods. An observational study over a four-year period on women undergoing a TVS prior to diagnosis of a tubal EP. Demographic details, presenting symptoms, TVS findings, serum hCG and progesterone levels were recorded at the time of the initial TVS and at the time of diagnosis of the EP in those initially classified as a PUL. Results. 411 women with a tubal EP underwent a TVS prior to treatment. In 85.9% (353/411) the EP was visualized on the initial TVS while 14.1%(58/411) were initially classified as PULs. Those initially classified as PULs had significantly lower mean gestational age and mean initial human chorionic gonadotrophin (hCG) levels, and significantly higher mean progesterone level at presentation than those where the EP was visualized on the initial TVS. Of those with a PUL, 60.3% (35/58) had the EP subsequently visualized on TVS. At the time of diagnosis these EPs were significantly smaller (p0.0001); the appearance of the EPs, serum hCG and progesterone levels at the time of visualization on TVS were not significantly different from those visualized on the initial TVS. Conclusion. In women with EPs who are initially classified as PULs, failure of visualization of the EP on the initial TVS is likely to be due to the fact that they are too small and probably too early in the disease process.