Pregnancy Due Date Calculator (EDD)

Looking for more details? Read our comprehensive EDD guide

What is the Estimated Date of Delivery (EDD)?

The Estimated Date of Delivery (EDD), also known as the expected date of confinement, is a prediction of the date a pregnant woman will deliver her baby. A normal pregnancy lasts between 37 and 42 weeks.

Naegele's Rule

This calculator uses a modified version of Naegele's rule to determine the EDD. The standard rule assumes a 28-day cycle and estimates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP).

                EDD = LMP + 280 days + (Cycle Length - 28)
              

Gestational Age

Gestational age measures how far along the pregnancy is. It is typically expressed in weeks and days (e.g., 20 weeks and 3 days). It's important to remember that gestational age includes the two weeks prior to ovulation, meaning pregnancy officially "begins" before conception occurs.

Clinical Significance

Accurate dating of the pregnancy is crucial for:

Scheduling appropriate prenatal screening and diagnostic tests.
Monitoring fetal growth over time.
Determining viability in the event of preterm labor.
Managing post-term pregnancies.

Limitations of LMP Dating

LMP-based dating assumes the woman has regular menstrual cycles and ovulates exactly 14 days before the start of the next cycle. It can be inaccurate if:

The patient has irregular periods.
The exact date of the LMP is unknown or forgotten.
Conception occurred while using hormonal contraceptives.
Bleeding occurred in early pregnancy that was mistaken for a period.

Note: Early first-trimester ultrasound (measuring Crown-Rump Length) is generally considered the most accurate method for establishing or confirming the gestational age.

Case Example

Patient: Last menstrual period started on January 1st.

Calculation: Naegele's rule adds 7 days to the LMP and subtracts 3 months (or adds 9 months). Jan 1st + 7 days = Jan 8th. + 9 months = October 8th.

Interpretation: Estimated Due Date is October 8th.

Frequently Asked Questions

How many babies are actually born on their EDD?

Only about 4% to 5% of babies are born exactly on their estimated due date. It's more of a guideline; normal delivery can happen anywhere between 37 and 42 weeks.

If the ultrasound date differs from the LMP date, which one is used?

Clinical guidelines suggest changing the EDD to the ultrasound-derived date if the discrepancy is large enough. For example, if a first-trimester ultrasound differs from the LMP date by more than 5 to 7 days, the ultrasound date is generally preferred.

References

American College of Obstetricians and Gynecologists (ACOG). Method for Estimating Due Date. Committee Opinion No. 700. Obstet Gynecol 2017;129:e150-4.

How to Use

Select whether you are calculating by Last Menstrual Period (LMP) or Ultrasound. Input the date, and for ultrasound, the exact gestational age on that date. The tool computes the EDD and current gestational age.