The best time for the first ultrasound is 7-8 weeks, when there is usually a fetal heart, menstruation is normal 28-30 days, more than 8 weeks there is no fetal heart, this fetus may have embryonic abortion. 7 weeks most of them should have a fetal heart, this time to do ultrasound has the following benefits: 1, can exclude ectopic pregnancy, this wait if ectopic pregnancy has the risk of rupture bleeding; 2, can see if the fetus is alive, check to see if there is a fetal heart. The more the week of pregnancy is, the more inaccurate the measurement will be, generally at 7-8 weeks the measurement is more accurate.