Expected Date of Delivery Calculation
The expected date of delivery (EDD) for a patient with a last menstrual period (LMP) of 12th August 2025 is 19th May 2026.
Calculation Method
The calculation of EDD is based on established guidelines from obstetric societies. According to the Society of Radiologists in Ultrasound consensus recommendations, gestational age (GA) is calculated using the first day of the last menstrual period and is reported as the number of weeks plus the number of days 1.
The standard calculation method follows these steps:
- Start with the first day of the LMP (August 12,2025)
- Add 7 days (August 19,2025)
- Subtract 3 months (May 19,2026)
- Add 1 year if necessary (already included in the above calculation)
This calculation assumes a pregnancy duration of 280 days (40 weeks) from the LMP, which is the traditional Naegele's rule.
Evidence-Based Considerations
Recent evidence suggests that the actual median pregnancy length may be slightly longer than the traditionally assumed 280 days:
- Research indicates that the median pregnancy length is approximately 283 days from LMP in women with regular menstrual cycles 2
- A population-based validation study of over 21,000 deliveries found a median pregnancy length of 283.03 days 2
However, for standardized clinical practice and consistency in documentation, the traditional calculation resulting in 280 days from LMP remains the standard approach used in clinical settings 3.
Important Clinical Notes
- Gestational age should always be reported as the number of weeks plus the number of days (e.g., "12 weeks 3 days") for standardized reporting 3
- Early ultrasound dating (ideally between 7-14 weeks) provides the most accurate dating with a standard deviation of ±7 days 3
- If there is a discrepancy between LMP dating and early ultrasound dating, clinical guidelines typically recommend using the ultrasound dating if the difference exceeds established criteria 1
- For women with irregular menstrual cycles or uncertain LMP, ultrasound dating becomes particularly important 4
Potential Pitfalls in EDD Calculation
- Assuming a standard 28-day menstrual cycle when the patient has a different cycle length can lead to inaccurate dating 5
- Relying solely on LMP without ultrasound confirmation can result in misclassification of preterm and post-term births 6
- Studies show that when using LMP alone, there is a higher rate of apparent post-term pregnancies (11.4%) compared to ultrasound dating (1.9%) 6
- Calendar-based methods tend to predict the EDD to be approximately 3 days too early on average 7
For this specific patient with an LMP of August 12,2025, the EDD is May 19,2026, based on the standard clinical calculation method. This date should be confirmed with an early ultrasound examination for the most accurate dating.