The last menstrual period is 4.20, and the expected date of delivery is usually January 27 of the following year. The estimation of the expected date of delivery should be carried out by a professional doctor, and the week of pregnancy should also be checked according to the size of the gestational sac or bud and the length of the head and buttocks as indicated by the early ultrasound examination of the pregnant woman. It is necessary to start counting from the first day of the last menstrual period, the number of days should be added with 7, and the month should be subtracted with 3 or added with 9. Therefore, for the last menstrual period of 4.20, the expected date of delivery is usually January 27th of the following year. After specifying the expected date of delivery, pregnant women must follow the doctor to carry out standardized obstetric examination, which can detect pregnancy complications or comorbidities as early as possible, such as gestational hypertension, gestational diabetes mellitus, intrauterine fetal developmental delay, etc., and give early intervention and treatment, but also early detection of fetal anomalies, which can ensure the quality of pregnancy and safeguard the safety of mothers and infants.