Gestational sac

The gestational sac is the earliest structure seen on transvaginal ultrasound in early pregnancy, usually visible from around four and a half to five weeks of gestation, or roughly two to three weeks after embryo transfer. Its presence inside the uterine cavity is the first evidence used to distinguish a pregnancy that is developing in the right place from a pregnancy of unknown location or ectopic pregnancy. Seeing a gestational sac does not by itself confirm a healthy, ongoing pregnancy; the clinical team will look for a yolk sac and, at a slightly later scan, an embryonic pole with cardiac activity before describing the pregnancy as viable.

Also known as: sac on scan, early pregnancy sac

Last reviewed: 9 September 2026. Written by the Antega Editorial Team. General information only — always follow the advice of your own clinic, doctor or pharmacist.