How to calculate the due date
May 20, 2025
Source: Cainiu Health
Dr. Zhang Lu
Introduction
Estimated due date refers to the anticipated date of delivery for a pregnant woman. Methods for calculating the estimated due date typically include the last menstrual period calculation, ultrasound estimation, basal body temperature curve analysis, fetal movement estimation, and fertilization date calculation. The estimated due dates obtained through these methods are only approximate ranges, and the actual delivery date may vary due to individual differences.
The due date refers to the estimated date of delivery for a pregnant woman. Commonly used methods to calculate the due date include the last menstrual period calculation, ultrasound estimation, basal body temperature curve method, fetal movement estimation, and fertilization date calculation. Detailed explanations are as follows:
1. Last menstrual period calculation: This is the most commonly used method. Generally, the due date totals 280 days. If a woman has regular menstrual cycles and remembers the exact date of her last menstrual period, the due date can be calculated starting from the first day of the last menstrual period by subtracting 3 months or adding 9 months, and adding 7 days. For example, if the last menstrual period was January 1, 2024, the estimated due date would be approximately October 8, 2024.
2. Ultrasound estimation: Through ultrasound examination, doctors can measure fetal developmental indicators such as biparietal diameter, crown-rump length, and femur length to estimate gestational age and thus calculate the due date. This method is particularly accurate in later stages of pregnancy and is an effective estimation method, especially for women with irregular menstrual cycles or those who have forgotten the date of their last menstrual period.
3. Basal body temperature curve method: If a pregnant woman consistently tracks her basal body temperature, the last day of the low-temperature phase on the basal body temperature curve can be considered as the ovulation day. The due date can then be estimated by adding 264–268 days, or 38 weeks, to the ovulation date. This method requires continuous basal body temperature records from the pregnant woman.
4. Fetal movement estimation: For first-time mothers, fetal movements usually begin around the 18th to 20th week of pregnancy, while women who have been pregnant before tend to feel fetal movements about two weeks earlier. Therefore, the due date can be estimated based on the date when fetal movements first appear. Specifically, for first-time mothers, the due date is calculated by adding 22 weeks to the date fetal movements first appear, while for women who have been pregnant before, the due date is calculated by adding 24 weeks to the date fetal movements first appear.
5. Fertilization date calculation: If the exact date of fertilization is known, the due date will be 38 weeks from the fertilization date. This method requires accurate knowledge of the fertilization date, which is usually determined using methods such as basal body temperature tracking, ovulation test strips, or other auxiliary techniques.
The due dates calculated using these methods are all approximate estimates, and the actual delivery date may vary due to individual differences. Therefore, pregnant women should undergo regular prenatal checkups and closely monitor the development of their baby.
