Ultrasound is a non-invasive test that does not affect the fetus because there is no radiation. However, if you have an ultrasound on day 36, you can only see the structure of the double ring of the gestational sac because the number of days is too small, and it is difficult to see the fluctuation of the primitive heart tube and the structure of the yolk sac. Therefore, if you do abdominal ultrasound examination to determine whether the pregnancy is normal or not, it is recommended to do the examination again at 6 weeks, when you can observe the structure of the yolk sac and the double ring structure of the gestational sac, you can observe the echo of the fetal bud and the fluctuation of the primitive heart tube, which is important for determining intrauterine pregnancy and can exclude ectopic pregnancy as well as pregnancy in other parts of the body.