Early pregnancy ultrasound can estimate the gestational week through the head-buttock diameter, and mid-pregnancy ultrasound can be used to determine the gestational age through the biparietal diameter, head circumference, abdominal circumference and so on, so as to deduce the date of conception. Early pregnancy, especially 11~13+6 weeks of pregnancy for ultrasound, can be measured through the fetal head-buttock diameter (CRL) to estimate the gestational age, more accurate, gestational week is equal to the head-buttock diameter + 6.5 values. In mid-pregnancy, i.e., at ≥14 weeks of gestation, ultrasound can be used to estimate gestational age by measuring the biparietal diameter, head circumference, abdominal circumference, femur length, and other indicators in a comprehensive manner. After the ultrasound examination, the report card will usually be labeled with the week of pregnancy as “some w, some d”, with w representing the number of weeks and d representing the number of days, so that the date of conception can be extrapolated according to the specific value. Pregnant women are advised to confirm the expected date of delivery and the date of conception according to the doctor’s projection.