"The double decidual sac sign (DDSS) is a useful feature on early pregnancy ultrasound to confirm an early intrauterine pregnancy (IUP) when the yolk sac or embryo is still not visualised. It consists of the decidua parietalis (lining the uterine cavity) and decidua capsularis (lining the gestational sac) and is seen as two concentric rings surrounding an anechoic gestational sac. Where the two adhere is the decidua basalis, and is the site of future placental formation."