Free tool · Calculator · L&D / OB
EDD & Gestational Age Calculator.
Get the estimated due date and current gestational age from a last menstrual period (Naegele’s rule) or from an ultrasound dating exam. Enter one or both — if you enter both, the tool compares them against the ACOG/AIUM/SMFM redating thresholds and tells you whether the LMP date should be changed to the ultrasound date. Calendar math only; the dating decision is your provider’s.
By last menstrual period (LMP)
Naegele’s rule: EDD = LMP + 280 days, adjusted for cycle length.
By ultrasound (optional)
Enter the scan date and the gestational age the ultrasound measured.
Enter an LMP and/or an ultrasound to see the due date and gestational age.
ACOG redating thresholds — when ultrasound replaces LMP [2]
| GA by ultrasound | Method | Redate if LMP–US differ by more than |
|---|---|---|
| ≤8w6d | CRL | 5 days |
| 9w0d–13w6d | CRL | 7 days |
| 14w0d–15w6d | BPD/HC/AC/FL | 7 days |
| 16w0d–21w6d | BPD/HC/AC/FL | 10 days |
| 22w0d–27w6d | BPD/HC/AC/FL | 14 days |
| ≥28w0d | BPD/HC/AC/FL | 21 days |
When the discrepancy exceeds the threshold for the gestational age at the time of the scan, the estimated due date is generally changed to the ultrasound date; otherwise the LMP date is kept. The earliest accurate ultrasound is the most reliable dating tool. The official dating decision is made by your provider.
References
- Naegele’s rule for the estimated date of delivery: EDD = first day of LMP + 280 days (40 weeks), assuming a 28-day cycle. Standard obstetric dating method.
- American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine, Society for Maternal-Fetal Medicine. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150–e154. acog.org. (Redating thresholds by gestational-age window.)
Thresholds transcribed from ACOG Committee Opinion 700. Your provider sets the dating of record.
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