Why Are Some Ectopic Pregnancies Characterized as Pregnancies of Unknown Location at the Initial Transvaginal Ultrasound Examination?

Abstract

The majority of ectopic pregnancies (EPs) initially classified as pregnancies of unknown location (PULs) are usually identified subsequently as either an early intrauterine pregnancy or a failed PUL. Some EPs not visualized on the initial transvaginal ultrasound scan (TVS) may have features that make them different from those that are visualized on the initial TVS. This prospective observational study compared the appearance and features of EPs initially classified as PULs with those that were visualized on the initial TVS examination. The study subjects were women attending an early pregnancy unit between 2002 and 2006 who underwent a TVS before diagnosis of a tubal EP. Women initially classified as a PUL at the time of the initial TVS and diagnosed later as EPs had serum human chorionic gonadotrophin (hCG) and progesterone levels recorded at the time of their initial TVS and subsequent TVS. Of the 411 women with a tubal EP who underwent TVS before treatment, 353 (85.9%) had the EP visualized on the initial TVS and 58 (14.1%) were initially classified as PULs. The mean gestational age and mean initial hCG levels were significantly lower, and the serum progesterone levels significantly higher among women initially classified with a PUL compared to women in whom the EP was visualized on the first TVS examination. Of the 58 women initially classified as PULs, the EP was visualized and diagnosed with subsequent TVS in 35 (61.4%). The mean size of the EPs at the time of diagnosis was significantly smaller than in those initially classified as PULs (P < 0.0001). No significant differences between the initial TVS examination and subsequent TVS were observed in the appearance of the EPs and in the mean serum hCG and progesterone levels. The investigators believe that the failure to visualize EPs on the initial TVS is most likely a result of their small size and scanning too early in their development.