Normally, the number of days of pregnancy can be calculated based on the last menstrual period, i.e. from the first day of the last menstrual period. If you have regular periods, the number of days to conceive is usually accurate and the growth and development of the fetus at the corresponding gestational week is generally consistent. If you have irregular periods, you cannot calculate the number of days of pregnancy by the last menstrual period and you need to calculate the number of days of pregnancy based on the ultrasound examination during pregnancy. The gestational sac is usually first detected by ultrasound at around 5 weeks of pregnancy; the germinal bud and primitive heart tube beat can be observed for the first time under ultrasound at around 7 weeks of pregnancy. Around 11-13 weeks of gestation, the gestational week can be accounted for by the ultrasound probe for breech length; beyond 14 weeks of gestation, the gestational week can be determined by a combination of biparietal diameter, head circumference, abdominal circumference and femoral length. In early pregnancy, it is recommended to have a timely check of the gestational week and to perform the appropriate pregnancy checkups according to the accurate gestational week.