Sonographic signs of early pregnancy.

The early embryo (blastocyst) is implanted about 6 to 7 days after fertilization and becomes completely imbedded within the decidua at 9.5 days. As the exocoelomic cavity (or early gestational sac) enlarges, it becomes visible by ultrasonography. The gestational sac, however, remains within markedly thickened ipsilateral decidua. The uterine cavity remains relatively undistorted (i.e., straight) and can be delineated by ultrasonography. The gestational sac, therefore, can be seen adjacent to a straight uterine cavity within a thickened decidua. This is called an intradecidual sign. Before the appearance of a visible gestational sac, an echogenic area within the thickened decidua may be seen. This is the earliest sign of intrauterine gestation, and it can be seen as early as 25-days menstrual age. The earliest evidence of an embryo is seen about 2 weeks later, when two small bubbles (amniotic and yolk sacs) attach to the wall of the gestational sac. The embryonic disc is located between the two bubbles. This is called the double-bleb sign. The intradecidual and double-bleb signs represent the most important signs of early intrauterine pregnancy.