There are only two data for the fetal sac usually because the sac is small and a third data cannot be measured. Fetal sac is generally referred to as the gestational sac, which is the primordial placental tissue, a small embryo wrapped by the amniotic membrane and vascular network, at this time, the fetus has not yet formed. When doing ultrasound, you can only observe that there is a gestational sac, and under normal circumstances, there will be three values of the gestational sac, indicating the length, width and height of the sac. If a pregnant woman has only two data on the result of the gestational sac when doing the examination, it is usually due to the incomplete development of the fetus and the small size of the gestational sac, which makes it impossible to measure the third data, therefore, only flat measurements can be made to give two data. Pregnant women do not need to be overly worried. They should follow the advice of their specialists, go to the hospital for regular check-ups, and if there is any abnormality, follow the doctor’s instructions for targeted treatment in a timely manner.