The size of the gestational sac in the first trimester cannot be generalized; the size of the developing gestational sac is positively correlated with the actual gestational week of pregnancy. Within 50 days of pregnancy, the average diameter of the gestational sac (in cm) plus 3.0 cm or the germ/head-rump length plus 6.5 cm is usually the gestational week of pregnancy. The gestational sac can usually be seen under ultrasound after 5 weeks of pregnancy and usually disappears by 10 weeks to form the placenta, the specific number of weeks and the corresponding size of the gestational sac are as follows: 1. 5 weeks: the embryo is about 0.4cm and the size of the gestational sac is about 0.5cm, the volume is less than 1/4 of the uterine cavity and can be seen under ultrasound at this time; 2. 6 weeks: the embryo grows to 0.85cm and the size of the gestational sac is about 1.2cm. The fetal sac is clearly visible on ultrasound, and the fetal bud and heartbeat can be seen; 3 and 7 weeks: the embryo grows to 1.33 cm, and the size of the gestational sac is 2.5-3.5 cm, which is more clearly visible on ultrasound. The size of the sac accounts for about 1/3 of the uterine cavity, and the fetal bud and heartbeat are also obvious; 4. 8-10 weeks: the size of the sac varies greatly depending on the basic height and weight of the pregnant woman, the level of diet and the amount of exercise during pregnancy, and the size of the sac lacks a clear reference value during this period, and the sac starts to disappear and gradually forms the placenta in the ultrasound in the 10th week.