The time of pregnancy is calculated from the first day of the last menstrual period, for example, if you have your period on the first day of January, you will be two months pregnant by the first day of March, which is eight weeks plus four days in clinical practice, which is the calculation time of pregnancy. If you have irregular periods, it is difficult to determine the time of ovulation, so clinically it is not calculated according to the time of the last period, but according to the first ultrasound examination, the number of weeks of pregnancy diagnosed by ultrasound, and based on this clinical projection of the expected date of delivery, which is scientifically based. Therefore, in pregnancy, the vast majority of women experience early pregnancy reactions, such as nausea, vomiting, anorexia and other gastrointestinal reactions, at around six weeks of pregnancy.