Intradecidual sign: a US criterion of early intrauterine pregnancy.

The uterine cavity appears on sonograms as a linear echo, which is usually visible during early pregnancy and remains straight until the eighth to ninth week of gestation. The early gestational sac is not enveloped by two layers of decidua, as suggested by descriptions of the double decidual sac sign; the sac (or echogenic area of early implantation) is actually located within a markedly thickened decidua on one side of the uterine cavity. The combination of these two sonographic characteristics is called the "intradecidual sign." An early implantation of 25 days gestational age can be detected by the presence of the intradecidual sign, which is sooner than a gestational sac can be seen. The implantation site can also be located by means of the intradecidual sign. In a study of 36 patients with early intrauterine pregnancy and five with ectopic pregnancy, the intradecidual sign was more sensitive (91.7% vs. 63.9%) and specific (100% vs. 60%) than the double decidual sac sign in the detection of early intrauterine pregnancy.