The general error of ultrasound in detecting the time of pregnancy is 1 to 2 weeks. The clinical projection of the time of pregnancy is based on the date of the first day of the last menstrual period, with every seven days counting as one week, and the entire pregnancy lasting about 40 weeks. This method of calculation is usually for women who usually have regular menstruation with a menstrual cycle of about 28 days. If menstruation is irregular, you will need to rely on ultrasound to correct the week of pregnancy. Ultrasound tests for pregnancy usually have an error of 1 to 2 weeks. Women may experience delayed implantation due to delayed ovulation or prolonged menstrual cycle, so the actual pregnancy week detected by ultrasound will be 1 to 2 weeks smaller than the clinical pregnancy week. There is no need to worry too much about this situation, just continue to do good obstetric monitoring.