Pregnancy Due Date: The Complete Clinical Guide
Naegele's rule, cycle-adjusted EDD, IVF dating, ACOG 2024 ultrasound guidelines, gestational age explained for doctors, midwives and expectant parents
What Is the Estimated Due Date (EDD)?
The estimated due date (EDD), also called the estimated date of confinement (EDC) or estimated date of delivery, is the projected date when a pregnant woman is expected to give birth. A normal human pregnancy lasts between 38 and 42 weeks, with the EDD representing the midpoint — 40 weeks (280 days) from the first day of the last menstrual period (LMP).
Only approximately 4–5% of babies are born on their exact calculated due date. Around 90% of deliveries occur within two weeks of the EDD. The date should always be understood as the centre of a probability window rather than a fixed endpoint, and should ideally be confirmed or revised by first-trimester ultrasound (ACOG Committee Opinion No. 700, Reaffirmed 2024).
Naegele's Rule — The Standard LMP Method
Equivalently: LMP − 3 months + 7 days + 1 year
Named after Franz Karl Naegele (1778–1851), the German professor of obstetrics at Heidelberg who published the formula in his 1830 midwifery textbook, Naegele's rule remains the global standard for LMP-based EDD calculation. The formula assumes a regular 28-day menstrual cycle with ovulation on day 14.
An important limitation: approximately 50% of women do not accurately recall their LMP date (ACOG 2017). Additionally, one study found that 40% of women undergoing first-trimester ultrasound had their EDD adjusted by more than 5 days compared to LMP-based dating. For these reasons, ACOG recommends that all pregnancies should have a first-trimester ultrasound to confirm or revise the EDD.
Cycle Length Adjustment
Naegele's rule assumes a 28-day cycle. For women with longer or shorter cycles, ovulation occurs later or earlier respectively, shifting the actual conception date. For example, a woman with a 35-day cycle ovulates approximately 7 days later than the standard assumption — so her EDD should be shifted 7 days later. Conversely, a woman with a 21-day cycle should have her EDD moved 7 days earlier.
This calculator automatically adjusts for cycle length — an important clinical feature particularly relevant for women with polycystic ovary syndrome (PCOS), irregular periods, or those tracking ovulation.
IVF and ART Dating — ACOG 2024 Guidance
Day 3 embryo transfer: EDD = Transfer date + 263 days
Day 5 blastocyst transfer: EDD = Transfer date + 261 days
For ART pregnancies, the embryo age at transfer is precisely known, making IVF-derived dating more accurate than LMP-based methods. The LMP should not be used to date ART pregnancies. Ultrasound confirmation is still recommended in the first trimester to assess embryo viability and confirm dates.
Ultrasound Dating — ACOG 2024 Guidelines
ACOG states that first-trimester ultrasound (up to 13+6 weeks) is the most accurate method to establish or confirm gestational age. CRL (crown-rump length) measurement in the first trimester has an accuracy of ±5–7 days (95% CI).
| Trimester / Timing | Measurement | Accuracy (95% CI) | Action if discrepancy |
|---|---|---|---|
| 1st trimester ≤13+6 wks | CRL (crown-rump length) | ± 5–7 days | Redate if discrepancy >5 days |
| 2nd trimester 14–27+6 wks | BPD, HC, AC, FL | ± 7–10 days | Redate if discrepancy >10 days |
| 3rd trimester ≥28 wks | BPD, HC, AC, FL | ± 21–30 days | Redate only in exceptional circumstances |
A pregnancy without an ultrasound examination that confirms or revises the EDD before 22+0 weeks should be considered suboptimally dated (ACOG 2024).
Gestational Age vs Fetal Age
Pregnancy Milestones & Key Clinical Dates
| Gestational Age | Milestone / Scan | Clinical Significance |
|---|---|---|
| 6–8 weeks | Early viability scan | Confirm intrauterine pregnancy, fetal heartbeat, CRL dating |
| 11–13+6 weeks | Dating scan + NT scan | CRL dating, nuchal translucency, chromosomal screening |
| 15–20 weeks | Anomaly / mid-trimester scan | Fetal anatomy survey, placental position, amniotic fluid |
| 28 weeks | Growth scan (if indicated) | 3rd trimester begins, anti-D if Rh negative |
| 37 weeks | Term begins | Early term (37–38+6 wks). Delivery appropriate if indicated. |
| 39–40 weeks | Full term | Optimal delivery window. Induction may be offered at 39 wks. |
| 41 weeks | Late term | Increased monitoring. Induction discussion. |
| ≥42 weeks | Post-term | Increased perinatal risk. Induction strongly recommended. |
Term Definitions (ACOG / RCOG)
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Explore All Tools →References
- ACOG Committee Opinion No. 700. Methods for Estimating the Due Date. Obstet Gynecol. 2017;129:e150-4. Reaffirmed 2024.
- ACOG Committee Opinion No. 579. Definition of Term Pregnancy. Obstet Gynecol. 2013;122:1139-40. Reaffirmed 2022.
- RCOG Green-top Guideline No. 31. The Investigation and Management of the Small-for-Gestational-Age Fetus. RCOG 2023.
- Whitworth M, Bricker L, Mullan C. Ultrasound for fetal assessment in early pregnancy. Cochrane Database Syst Rev. 2015;(7):CD007058.
- Naegele FK. Lehrbuch der Geburtshuelfe. Mainz: Kupferberg; 1830.
- Savitz DA et al. Comparison of pregnancy dating by LMP, ultrasound, and combination. Am J Obstet Gynecol. 2002;187:1660-6.
- Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. Am J Obstet Gynecol. 1996;174:278-81.
- Society for Maternal-Fetal Medicine. Ultrasound: Dating Criteria and Worksheet. Updated June 2024.
⚠️ Medical Disclaimer: This calculator provides EDD estimates for educational and clinical documentation purposes only. All dates should be confirmed with first-trimester ultrasound by a qualified obstetric provider. This tool does not replace professional obstetric assessment or antenatal care.