Result
How to use this calculator
- LMP is the most common method — pick the first day of your last period.
- IVF transfer dates: be sure whether your transfer was 3-day or 5-day embryo (your clinic told you).
- Conception dates are only reliable if you have ovulation tracking (BBT or LH strips).
- Always confirm with your doctor — first-trimester ultrasound dating is more accurate.
About this tool
A pregnancy lasts about 40 weeks counted from the first day of the last menstrual period — a calculation called Naegele's rule, used since the 19th century. If you know the conception date instead (often the case after IVF or with carefully tracked cycles), the math shifts by 14 days. Only about 5 % of babies arrive on the exact predicted day; "term" is anywhere from 37 to 42 weeks. The due date is mainly a planning anchor for prenatal appointments, not a deadline. An ultrasound between weeks 8 and 14 gives a more accurate date than LMP alone.
How it works — the formula
Naegele's rule: EDD = LMP + 280 days (= LMP + 7 days − 3 months + 1 year)
Ultrasound-dated: EDD adjusted from CRL measurement (gold standard in T1)Naegele's rule (Franz Karl Naegele, 1830) estimates the estimated delivery date as 280 days from the last menstrual period, assuming a 28-day cycle with ovulation on day 14. The American College of Obstetricians and Gynecologists (ACOG) and the American Institute of Ultrasound in Medicine (AIUM) jointly recommend first-trimester ultrasound dating using crown-rump length (CRL) as the most accurate method; LMP dating is used when ultrasound is unavailable or significantly disagrees by less than 7 days.
Worked examples
- Inputs:
- LMP = 2026-01-15
- Output:
- EDD ≈ 2026-10-22 (LMP + 280 days)
- Inputs:
- LMP = 2026-01-15, cycle = 32 days
- Output:
- EDD ≈ 2026-10-26 (+4 days vs 28-day baseline)
- Inputs:
- Day-5 transfer 2026-02-01
- Output:
- EDD ≈ LMP-equivalent + 280 d, with conception fixed by transfer date
Limitations
- Naegele's rule assumes a 28-day cycle with ovulation on day 14; real cycles vary by ±5 days, shifting the EDD accordingly.
- Only ~5% of pregnancies deliver on the exact EDD; the spontaneous-labor distribution spans roughly 38–42 weeks.
- First-trimester ultrasound (CRL) is the gold standard; LMP dates that differ by more than 5–7 days from ultrasound should be revised per ACOG 700.
- IVF dates are exact — use the embryo-transfer date plus age-of-embryo to calculate EDD, not LMP.
Due-date calculations are estimates only. This calculator does not provide medical advice — your obstetric care team's ultrasound-confirmed EDD is the operative date for clinical decisions.