Intradecidual sign | Radiology Signs

Answer:

The intradecidual sign is a small fluid collection embedded eccentrically within the thickened decidua, adjacent to but not deforming the central endometrial stripe, and it is the earliest sonographic evidence of an intrauterine pregnancy, visible from about 4.5 weeks and before the yolk sac appears. Its value is in the woman with a positive pregnancy test, a low hCG and an otherwise empty-looking uterus, where it argues for an early intrauterine gestation rather than an ectopic pregnancy. Two cautions apply. First, its interobserver reliability is moderate at best, and studies comparing it with the double decidual sac sign found neither to be consistently reproducible, so it should support rather than settle the diagnosis. Second, a collection lying centrally in the cavity, distorting or splitting the endometrial stripe, or with an irregular margin, is more likely to be a pseudogestational sac or a small haematoma, and in the wrong context that misinterpretation is dangerous. Current practice classifies a round or oval intrauterine collection in a pregnant woman as a probable gestational sac and manages a pregnancy of unknown location with serial hCG measurement and rescanning, using strict criteria for non-viability before intervening, so that a viable early pregnancy is never terminated on the basis of an equivocal early scan.

Why is it called so?

The sac is seen within the decidua itself rather than in the endometrial cavity, hence intradecidual.

Pathophysiology

Implantation occurs when the blastocyst burrows into the endometrium, so the earliest gestational sac lies within the decidualised stroma on one side of the cavity rather than in the cavity lumen. At this stage the chorionic cavity is only a few millimetres across and is surrounded by a rim of trophoblast that is echogenic against the decidua. Because the sac is eccentric and small, the central endometrial stripe formed by the apposed anterior and posterior endometrial surfaces remains visible and undisplaced. A pseudosac, by contrast, is fluid within the cavity itself, so it lies centrally and separates the two endometrial surfaces, which is why position relative to the stripe is the anatomical discriminator.

Alternative names: Intradecidual sac sign

Other associated named signs: Double decidual sac sign, the yolk sac as the first definite structure and the pseudogestational sac

References

1. Yeh HC, Goodman JD, Carr L, Rabinowitz JG. Intradecidual sign: a US criterion of early intrauterine pregnancy. Radiology. 1986;161(2):463-7. PMID: 3532191.