The gestational sac can be seen by ultrasound as early as about 5 weeks after pregnancy, and about 3-4 days earlier by negative ultrasound. If you want to see it earlier, you can use negative ultrasound, if it is clear whether there is fetal heart and fetal bud, it is recommended to wait until about 7 weeks for ultrasound to see whether the gestational sac is inside or outside the uterus, and whether there is fetal heart, fetal bud and yolk sac. If the fetal heart, fetal bud and yolk sac can be seen in the uterus, it proves that the pregnancy is intrauterine and the fetus is good, so you can continue to observe until about 12 weeks of pregnancy to do ultrasound to see the thickness of the nuchal translucency layer of the fetus.