How accurate is transvaginal us in early pregnancy?

US is not typically used to diagnose pregnancy unless the patient presents with vaginal bleeding or abdominal pain early in gestation or is a high-risk obstetric patient.

With the advent of transvaginal US (TVUS), the diagnosis of pregnancy can be made even earlier than is possible with transabdominal US (TAUS).

TVUS is the most accurate means of confirming intrauterine pregnancy and gestational age during the early first trimester. TVUS can help detect signs of intrauterine pregnancy approximately 1 week earlier than TAUS. Patients are not required to have a full bladder and are not required to endure uncomfortable pressure on the abdominal wall from the external probe. TVUS is also better for patients who are obese or those who guard during TAUS. One disadvantage is that some patients are anxious about the transvaginal probe and may object to its insertion. The earliest structure identified is the gestational sac. The GS can be seen on TVUS images by 4-5 weeks' gestation and grows at a rate of 1 mm/d in early gestation. By 5.5-6 weeks' gestation, a double-decidual sign can be seen, which is the GS surrounded by the thickened decidua. The presence of an early GS can be confused with a small collection of fluid or blood or the pseudo GS of an ectopic pregnancy. Because of this, the diagnosis of intrauterine pregnancy should not be made on the basis of visualization of the GS alone.

The diagnosis of intrauterine pregnancy can be made once the yolk sac is present, which also excludes ectopic pregnancy, except in the rare instance of heterotopic pregnancy.

The fetal pole is first seen on TVUS images at approximately 5-6 weeks' gestation. It should always be seen by TVUS when the GS is larger than 18 mm or by TAUS when the GS is larger than 2.5 cm. The fetal pole is a linear hyperechoic structure that grows at approximately 1 mm/d.