Prospective comparison of vaginal and abdominal sonography in normal early pregnancy

Vaginal and abdominal sonography were prospectively compared in 309 consecutive pregnancies of which 175 were normal. Two sonographic criteria of normal were analyzed: (1) the presence of an embryo compared with average gestational sac size and (2) the presence of embryonic cardiac activity compared with crown-rump length. Vaginally, 100% (160/160) of embryos were visualized when the average sac diameter was greater than or equal to 12 mm. Abdominally, an embryo was noted in 99.2% (123/124) of gestational sacs greater than or equal to 27 mm. Vaginal sonography revealed cardiac motion in all embryos of greater than or equal to 5 mm (149/149 cases); abdominal sonography revealed cardiac activity in 100% (132/132) of embryos with a crown-rump length of greater than or equal to 9 mm. One-third of normal embryos less than 5 mm crown-rump length did not demonstrate cardiac activity. The study demonstrates through objective comparison that vaginal sonography is superior to abdominal sonography for detection of an intrauterine embryo and its cardiac activity before 8 menstrual weeks. The diagnosis of embryonic demise should not be made by vaginal sonography in embryos measuring less than 5 mm crown-rump length without a heartbeat, and an empty gestational sac of less than 12 mm average diameter should not be diagnosed as blighted ovum by vaginal scans. In these cases follow-up vaginal sonography is suggested.