How to Calculate the Estimated Date of Delivery
May 20, 2025
Source: Cainiu Health
Dr. Zhang Lu
Introduction
Calculating the due date helps pregnant women prepare for childbirth. Methods for calculating the due date typically include estimating based on the last menstrual period, the date of first fetal movement, the basal body temperature curve, the onset date of morning sickness, and ultrasound examination results. However, the due date is only an estimated date of delivery, and the actual delivery date may vary due to various factors.
Calculating the due date helps pregnant women and their families prepare for childbirth. Common methods for calculating the due date include calculating based on the last menstrual period, fetal movement date, basal body temperature curve, date of morning sickness, and ultrasound examination. Detailed explanations are as follows:
1. Calculation based on the last menstrual period: This is a commonly used method. The pregnant woman needs to accurately remember the first day of her last menstrual period. Add 9 months or subtract 3 months from the month of the last menstrual period, and add 7 days to the date of the last menstrual period to determine the specific due date. For example, if the last menstrual period was January 1st, the due date would be approximately October 8th.
2. Calculation based on fetal movement date: If the pregnant woman has forgotten the date of her last menstrual period, the due date can be estimated based on the date of first fetal movement. Fetal movement generally begins 18–20 weeks after pregnancy. For primiparas, the estimated due date is approximately 20 weeks after the first fetal movement, while for multiparas, it is approximately 22 weeks after the first fetal movement.
3. Calculation based on the basal body temperature curve: If the pregnant woman consistently measures her basal body temperature, the due date can be calculated based on the basal body temperature curve. The last day of the low-temperature phase on the curve is considered as the day of ovulation. Adding 264–268 days to the ovulation date will yield the estimated due date.
4. Calculation based on morning sickness date: Morning sickness typically occurs around the sixth week of pregnancy. Adding 280 days to the onset date of morning sickness gives the estimated due date. However, this method is relatively less accurate because of significant individual differences in morning sickness symptoms.
5. Estimation based on ultrasound examination: For women with irregular menstrual cycles or those who have forgotten the date of their last menstrual period, the due date can be estimated through ultrasound examination. Doctors estimate fetal age based on fetal development indicators such as biparietal diameter, crown-rump length, and femur length, and then calculate the due date accordingly.
However, the due date is only an estimated date for delivery, and the actual delivery date may vary due to various factors. Pregnant women should attend regular prenatal checkups at the hospital to monitor fetal development and their own health condition.
