The gestational sac first becomes apparent on trans-vaginal ultrasound (TVUS)

The gestational sac first becomes apparent on trans-vaginal ultrasound (TVUS) at approximately 4.5-5 weeks of gestational age, appearing as a round anechoic structure located eccentrically within the echogenic decidua.

Subsequent to the appearance of the gestation sac, two concentric echogenic rings encircling the central anechoic collection develop: the outer ring represents the decidua parietalis, while the inner ring represents the decidua capsularis and chorion. This is known as the double decidual sac sign (DDS), which is a definitive sign of an intrauterine pregnancy (IUP).

At around 5.5 weeks of gestation, the developing yolk sac becomes visible. Initially appearing as two echogenic parallel lines at the periphery of the gestational sac, the yolk sac eventually acquires its typical round appearance by the end of 5.5 weeks.

The embryo (sometimes referred to as the fetal pole early on) becomes apparent at 6 weeks of gestation as a relatively featureless echogenic linear or oval structure adjacent to the yolk sac, initially measuring 1-2 mm in length. At this point, the mean gestational sac diameter (MSD) is approximately 10 mm.

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