How to Calculate the Estimated Date of Delivery (Formula)

May 08, 2025 Source: Cainiu Health

Dr. Zhang Lu

Introduction

In general, the estimated due date can be calculated using the last menstrual period (LMP) method. For pregnant women with regular menstrual cycles of 28-30 days, the calculation formula is as follows: Estimated due date month = Last menstrual period month + 9 or - 3 Estimated due date day = Last menstrual period day + 7 For example, if the last menstrual period was May 10, 2024: Month: 5 (May) + 9 = 14

Generally, the estimated due date can be calculated using the last menstrual period (LMP) method. The formula is as follows: Estimated due date month = LMP month + 9 or - 3; estimated due date day = LMP day + 7. The detailed explanation is as follows:

For pregnant women with regular menstrual cycles of 28-30 days, the calculation formula is: Estimated due date month = LMP month + 9 or - 3; estimated due date day = LMP day + 7. For example, if the last menstrual period was May 10, 2024, the month calculation would be 5 (May) + 9 = 14, which exceeds 12, so it would be February of the following year. The day calculation would be 10 + 7 = 17. Therefore, the estimated due date would be February 17, 2025. This formula applies to most cases, but the actual due date may vary due to individual differences and different ovulation times.

If a pregnant woman has irregular menstrual cycles, the estimated due date can also be determined using ultrasound examination. In early pregnancy, doctors may perform an ultrasound to measure fetal size and thereby estimate gestational age and calculate the estimated due date. The main ultrasound parameters include crown-rump length (CRL) and biparietal diameter (BPD), with corresponding reference values for different gestational ages. For example, if an early pregnancy ultrasound shows a fetal CRL of 5.0 cm, the formula for estimating gestational age based on CRL is: gestational weeks = CRL (cm) + 6.5, which gives approximately 11.5 weeks. By calculating the corresponding number of weeks from the examination date, the estimated due date can be determined.

The estimated due date is only an approximation, and the actual delivery date may be earlier or later.

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