Clinical Due Date Calculator
Estimate your baby's arrival date using your last menstrual period, known conception date, or first-trimester ultrasound biometric measurements with automated cycle-length reconciliation.
Interactive Due Date Calculation Tool
The Science & Clinical Accuracy of Due Date Estimation
A calculated estimated date of delivery (EDD) represents the completion of 40 completed gestational weeks (280 days from the onset of your last menstrual period). However, according to the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM), spontaneous human parturition is a biological distribution spanning weeks 37 through 42 rather than an exact pinpoint calendar day.
Clinical Classification of Gestational Delivery Windows (ACOG / SMFM)
| Clinical Classification | Gestational Age Window | Spontaneous Birth % | Clinical Significance |
|---|---|---|---|
| Preterm | < 37 weeks 0 days | ~8–10% | Requires specialized neonatal care and respiratory evaluation. |
| Early Term | 37 weeks 0 days – 38 weeks 6 days | ~26% | Mature organ systems; continued rapid pulmonary and brain development. |
| Full Term (Ideal) | 39 weeks 0 days – 40 weeks 6 days | ~55–60% | Optimal neonatal outcomes, lowest incidence of respiratory distress. |
| Late Term | 41 weeks 0 days – 41 weeks 6 days | ~6–8% | Requires biophysical profile (BPP) and amniotic fluid volume testing. |
| Postterm | ≥ 42 weeks 0 days | < 2% | Routine clinical labor induction recommended to mitigate placental insufficiency. |
Why Do Different Dating Methods Yield Discrepant Dates?
- Last Menstrual Period (LMP): Naegele’s traditional rule relies on a 28-day cycle with ovulation on day 14. In patients with 35-day cycles, ovulation typically occurs on day 21, shifting the true physiological due date 7 days later.
- Conception Date Method: When conception is pinpointed via ovulation predictor kits, basal body temperature charting, or intra-uterine insemination (IUI) / IVF blastocyst transfer, clinical dating deterministically adds 266 days (38 weeks), bypassing cycle length variability.
- First-Trimester Ultrasound (ACOG Gold Standard): Measures fetal Crown-Rump Length (CRL). In early embryonic stages, biological variation across all human fetuses is minimal (±3 to 5 days). If an ultrasound before 9 weeks differs from your LMP date by >5 days, the ultrasound date is universally adopted as the official clinical EDD.
Clinical Accuracy & Medical Disclaimer
This calculator provides educational gestational estimates based on standard obstetric formulas (such as Naegele's Rule and 280-day gestational conventions). Only ~4–5% of babies deliver precisely on their calculated due date; 80% deliver within 37 to 42 completed gestational weeks. This tool does not diagnose medical conditions or substitute for prenatal ultrasounds and professional obstetric guidance.
Related Pregnancy Calculators & Articles
Primary Pregnancy Calculator
Track gestational weeks, days, trimesters, and weekly anatomical milestones.
Conception Date Calculator
Work backward from your due date to discover conception timing and fertile windows.
Due Date Accuracy Guide
Explore clinical studies on spontaneous labor distributions and delivery windows.
Calculation Formulas Guide
Detailed clinical mathematics of Naegele's Rule and CRL ultrasound scans.