Estimated Due Date Calculation
For a patient with a regular 28-day menstrual cycle, the estimated due date (EDC/EDD) is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). 1
Standard Calculation Method
- Gestational age is calculated from the first day of the LMP, with the EDC set at 40 weeks (280 days) from that date. 1
- The calculation assumes ovulation occurs at day 14 of a 28-day cycle, with conception occurring approximately 2 weeks after the LMP. 2
- Gestational age should be reported as weeks and days (e.g., "28 weeks 3 days") rather than rounded weeks for optimal accuracy. 1, 3
Practical Application
To calculate the EDD from [DATE]:
- Add 280 days to the first day of the LMP 1
- Alternatively, use Naegele's rule: add 7 days to the LMP, subtract 3 months, and add 1 year 2
- Electronic calculators are recommended over manual gestational age wheels, as manual devices show significant bias with individual differences of up to 4 days and variability throughout the year. 4
Critical Assumptions and Limitations
- This calculation is only valid for patients with regular 28-day menstrual cycles. 2
- For irregular cycles (e.g., 35-day cycle), you must adjust by adding the difference between the patient's cycle length and 28 days to account for delayed ovulation. 2
- For a 35-day cycle, ovulation typically occurs around day 21 (7 days later than standard), requiring an additional 7 days to be added to the calculated due date. 2
When to Modify This Approach
First-trimester ultrasound with crown-rump length (CRL) measurement between 8-13 weeks provides superior accuracy (±5-7 days) compared to LMP-based dating and should be used when available. 2, 3
- When a discrepancy of ≥5 days exists between LMP and CRL dating, adopt the CRL-based gestational age for all clinical decision-making. 2, 3
- Never average LMP and ultrasound dates—always use the ultrasound date when available in the first trimester. 3
Special Circumstances to Avoid
- For pregnancies from assisted reproductive technology, LMP dating must not be used; instead, calculate based on embryo-transfer date and developmental stage. 1
- Do not use this calculation if the LMP is uncertain, the patient has irregular cycles, or recent hormonal contraception use affects cycle regularity. 2
Clinical Importance of Accurate Dating
- Accurate dating is crucial because maternal serum AFP levels increase 10-15% per week in the second trimester, and incorrect dating can lead to misinterpretation of screening results. 2, 3
- Proper dating affects the timing of all prenatal screening tests, Down syndrome screening, and neural tube defect screening. 3
- Research shows that the median time to non-elective delivery is actually 283 days after LMP (not 280), with nulliparous women delivering at 284 days versus 282 days for multiparous women. 5