Ultrasound features and significance of an abnormal gestational sac
On US examination, an irregularly shaped gestational sac that is low-lying and surrounded by a weakly echogenic and thin rim of decidual reaction is 90% to 100% predictive of an abnormal intrauterine pregnancy. A mean gestational sac diameter that is less than 5 mm greater than the embryonic CRL, which is known as first-trimester oligohydramnios, is also associated with a poor pregnancy outcome. A mean gestational sac size of 16 to 24 mm without an embryo is suspicious for a failed pregnancy. However, a higher cutoff of 25 mm is now recommended as the mean gestation sac size by which an embryo should be seen, before diagnosing a failed pregnancy.