If a woman’s menstrual period is irregular and ovulation is late, the number of days of pregnancy is mainly based on the ultrasound as the standard. If the embryo development is normal, according to the first ultrasound prompting the size of the gestational sac to project the patient’s actual pregnancy weeks, through the weeks of pregnancy to project the patient’s expected date of delivery. For example, the usual patient’s menstrual cycle is 40-60 days, the patient’s last menstrual period was July 30, 2019, the size of the gestational sac at the ultrasound prompt on September 30, 2019 was 1.0 x 1.5 x 1.2 cm, which is used to project the patient’s actual days of pregnancy as 42 days, but according to the patient’s time of menopause to project, the patient should be 60 days pregnant. Therefore, the patient needs to continue to observe, and if the embryo develops normally during the subsequent observation, the size of the gestational sac indicated by the patient’s first ultrasound will be used as the standard for calculating the number of weeks of the patient’s pregnancy.