Double decidual sac sign | Radiology Signs

Answer:

The double decidual sac sign is the appearance of an early gestational sac surrounded by two concentric echogenic rings separated by a thin hypoechoic layer, formed by the decidua capsularis with the chorion on the inside and the decidua parietalis of the opposite uterine wall on the outside. It supports the presence of an intrauterine pregnancy and was traditionally used to distinguish a true gestational sac from the pseudogestational sac of an ectopic pregnancy, which is a central collection of fluid or blood within the endometrial cavity with a single surrounding layer, often with an irregular outline and no eccentric implantation. Modern practice treats the sign as supportive but not definitive, because interobserver agreement is only moderate and it is absent in a substantial proportion of confirmed intrauterine pregnancies; the definitive findings are a yolk sac or an embryo within the sac. Current guidance is that a round or oval intrauterine fluid collection in a woman with a positive pregnancy test is a gestational sac with high probability, and that a pregnancy of unknown location is managed with serial hCG and repeat scanning rather than by pressing the sign. The intradecidual sign, an eccentrically located sac abutting the endometrial stripe, is the earlier finding on the same continuum.

Why is it called so?

The two decidual layers surrounding the sac produce two rings, so the sac appears to be double-walled.

Pathophysiology

After the blastocyst implants within the endometrium, the decidua adjacent to and covering it, the decidua capsularis, is separated from the decidua lining the remainder of the cavity, the decidua parietalis, by the residual endometrial cavity, which contains a small amount of fluid or mucus. Ultrasound resolves the two decidual layers as echogenic bands and the fluid between them as a thin hypoechoic line, giving the double ring. As the pregnancy grows, the sac expands to fill the cavity and the two layers fuse by around 12 weeks, so the sign disappears. In an ectopic pregnancy the endometrium thickens under hormonal stimulation and may accumulate fluid centrally, but only one decidual layer surrounds it, which is what generates the single-ring pseudosac.

Alternative names: Double sac sign; double ring sign

Other associated named signs: Intradecidual sign, the pseudogestational sac and the yolk sac sign of a definite intrauterine pregnancy

References

1. Bradley WG, Fiske CE, Filly RA. The double sac sign of early intrauterine pregnancy: use in exclusion of ectopic pregnancy. Radiology. 1982;143(1):223-6. PMID: 7063730.