Estimated Delivery Date Calculation
For a patient who was 8 weeks pregnant on the specified date, the estimated delivery date (EDD) is calculated by adding 32 weeks (224 days) from that date, as standard pregnancy duration is 280 days from the last menstrual period. 1
Standard Dating Methodology
The calculation follows Naegele's rule, which adds 280 days to the first day of the last menstrual period (LMP), though this should be considered a guideline rather than an absolute date. 2 Since the patient was already 8 weeks (56 days) pregnant at the reference date, the remaining gestational period is 32 weeks (280 - 56 = 224 days). 1
Accuracy Considerations
Ultrasound dating in early pregnancy (8-12.5 weeks) is more accurate than LMP-based calculations, with a prediction error of only 7.3 days. 3 At 8 weeks gestation, crown-rump length measurement provides the most precise dating, superior to calculations based on menstrual history alone. 3
When ultrasound dating is used instead of LMP alone, the number of incorrectly classified postterm pregnancies decreases from 10.3% to 2.7%. 3
The American College of Obstetricians and Gynecologists emphasizes that gestational age and EDD should be determined as soon as data from LMP, first accurate ultrasound, or both are obtained, and documented clearly in the medical record. 1
Important Clinical Caveats
A pregnancy without ultrasound confirmation or revision of the EDD before 22 0/7 weeks should be considered suboptimally dated. 1 Multiple factors can influence actual pregnancy length, including ethnicity, maternal height, parity, maternal weight, and variations in menstrual cycle timing. 2
Manual gestational age calculators (pregnancy wheels) have been shown to have significant bias, with individual errors up to 4 days, and should be validated before use. 4
Electronic calculators consistently demonstrate no error and are recommended over manual devices. 4
The EDD represents the best obstetric estimate for clinical care purposes and should be recorded on the birth certificate. 1