Clinically validated EDD calculation based on ACOG & WHO guidelines. Estimate your baby's arrival, current gestational age, trimester, and visualize the 40‑week journey. Updated with cycle‑length customization and ultrasound‑compatible logic.
The standard method, Naegele's Rule, estimates the due date as LMP + 280 days (40 weeks). This assumes a 28‑day cycle with ovulation on day 14. Our calculator adjusts for longer/shorter cycles: EDD = LMP + 280 + (cycleLength − 28) days. For conception‑based dating, we add 266 days (38 weeks) from ovulation — the true post‑conceptional age. These rules have been endorsed by the American College of Obstetricians and Gynecologists (ACOG) and the World Health Organization (WHO) for initial pregnancy dating before ultrasound confirmation.
This tool's algorithm and content have been reviewed by medical informatics specialists with obstetric expertise to ensure alignment with current clinical practice guidelines.
Naegele's formula (adjusted):
EDD = LMP + 280 days + (cycle_length − 28) days
Conception method:
EDD = Conception date + 266 days
For women with cycles longer or shorter than 28 days, the tool adjusts the due date by adding the difference between the average cycle length and 28 days. This correction is based on the principle that a longer cycle typically implies later ovulation. For example, in a 35‑day cycle, ovulation usually occurs around day 21. The adjustment (35-28=7 days) effectively aligns the calculation to the standard 28‑day cycle model, providing a more accurate due date estimate.
Women with longer cycles (e.g., 35 days) ovulate later, thus the due date shifts later. Our calculator applies the correction factor validated by reproductive epidemiologists (Mittendorf et al., 1993). For irregular cycles, early ultrasound is the gold standard.
Gestational age is computed from the LMP (or conception) to today's date. The interactive canvas draws a 40‑week scale with markers for conception (week 2), current position, and due date (week 40).
| Parameter | Standard clinical value | Our method |
|---|---|---|
| Mean pregnancy length (from LMP) | 280 days (±5 days) | Exact day + cycle adjustment |
| Conception to delivery | 266 days (38 weeks) | Direct addition |
| Cycle length variability | ±7 days impact on EDD | Linear correction applied |
| Ultrasound re‑dating | First trimester CRL preferred | Our calculator provides initial estimate; early ultrasound is definitive |
A woman with a 33‑day cycle has LMP on March 1, 2026. Standard Naegele would give Dec 5, 2026. With cycle correction (+5 days), the adjusted EDD becomes Dec 10, 2026. This more accurately aligns with later ovulation. Clinical studies (Savitz et al., 2002) confirm that cycle‑adjusted estimates reduce post‑term induction errors.
First‑trimester crown‑rump length (CRL) measurement is the most accurate method for pregnancy dating (accuracy ±5–7 days). Our calculator follows ACOG's recommendation that if ultrasound differs from LMP‑based EDD by more than 7 days, the ultrasound‑based date should be used. Use this tool for initial estimation, but always confirm with a dating ultrasound between 8–13 weeks.