How to Calculate the Number of Days of Pregnancy

There are usually several methods to calculate the number of days of pregnancy:

1. Last menstrual period (LMP) method: This is the most commonly used method. The calculation starts from the first day of the last menstrual period, with the entire pregnancy lasting approximately 280 days, or 40 weeks. For example, if the first day of the last menstrual period was August 1st, the estimated due date would be approximately May 8th of the following year (add 9 months or subtract 3 months, and add 7 days to the date).

2. Ultrasound examination method: An ultrasound examination is performed in early pregnancy (especially between 11-13 weeks of gestation), and the gestational age and number of pregnancy days are estimated by measuring fetal parameters such as crown-rump length. This method is more accurate for women with irregular menstrual cycles or unclear dates of the last menstrual period.

For example, some women have longer menstrual cycles, and the calculated number of pregnancy days based on the last menstrual period may differ from the results obtained via ultrasound. In such cases, doctors will rely on the ultrasound findings to reassess the gestational age.

Different calculation methods may have a certain margin of error, but generally, a difference of about one week is considered normal. During pregnancy, doctors will comprehensively evaluate fetal development and gestational age based on multiple factors.