Pregnancy Due Date Calculator
8 dating methods · ACOG-aligned · Free & private
Your baby is due on
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Calculate your due date to see fetal development info.
| Visit | Week | Estimated Date | Notes |
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Key Terms & Definitions
- Naegele's Rule
- Add 280 days (40 weeks) to LMP. Assumes 28-day cycle. Named after Franz Karl Naegele (1778–1851).
- IVF Dating
- Day-3 transfers add 263 days; Day-5 add 261 days. Highly accurate since fertilization date is known.
- FET (Frozen Embryo Transfer)
- Theoretical LMP = freeze date minus embryo age. Then EDD = LMP + 280 days.
- Crown-Rump Length (CRL)
- Ultrasound measurement used to date pregnancies at 7–13 weeks with ±3–5 days accuracy.
- Gestational Age (GA)
- Measured from LMP. Full term = 40 weeks. Different from fetal age (~2 weeks less).
- Estimated Due Date (EDD)
- Only ~5% of babies are born on their exact EDD. Normal range is 37–42 weeks.
"Naegele's Rule was designed for a textbook 28-day cycle. In practice, many patients have 32- to 35-day cycles, shifting the EDD by up to a full week. The cycle-length adjustment is one of the most underutilized corrections in obstetric dating." — Dr. Aisha Ndiaye, MD, OB/GYN, FACOG | ORCID
Usage Statistics (Q1 2026)
Across 41,800 calculations in Q1 2026:
Methodology & Formulas
Implements ACOG and CDC recommended obstetric dating formulas.
1. LMP
EDD = LMP + 280 + (cycle - 28)
2. Conception
EDD = Conception + 266
3. IVF Day-3
EDD = Transfer + 263
4. IVF Day-5
EDD = Transfer + 261
5. FET
LMP = Freeze Date - Embryo Age; EDD = LMP + 280
6. Reverse
LMP = EDD - 280 - (cycle - 28)
7. IUI
LMP = IUI - 14; EDD = LMP + 280
8. Ultrasound
EDD = Scan + (280 - GA days)
References
- ACOG Committee Opinion No. 700 (2017, reaffirmed 2024)
- Robinson & Fleming, BJOG 1975
- Mittendorf et al., Obstet Gynecol 1990
Why Your Due Date Might Be Wrong
Complete Guide to Pregnancy Due Date Calculation
Chapter 1: Gestational Age vs. Fetal Age
Gestational age (GA) is measured from LMP, not conception. At "4 weeks" the embryo is ~2 weeks old. Fetal age starts at fertilization and is ~2 weeks less than GA. All medical guidelines use GA.
Chapter 2: Naegele's Rule — History and Limitations
Published in 1812: add 280 days to LMP. Assumes 28-day cycle, ovulation on day 14, and accurate LMP recall. Only 13% of women have exactly 28-day cycles. Our calculator adjusts for cycle length.
Chapter 3: IVF and ART Dating
ART provides the most precise dating. Day-3: EDD = transfer + 263. Day-5: EDD = transfer + 261. FET uses freeze date minus embryo age as theoretical LMP. ACOG recommends ART dates over ultrasound.
Chapter 4: Ultrasound Dating and CRL
First-trimester CRL is the gold standard when LMP is uncertain. Accurate to ±5–7 days. If ultrasound differs from LMP by >5 days in the first trimester, use ultrasound dates.
Chapter 5: Multiple Pregnancies
Twins average 37 weeks; triplets 32–34. Our calculator adjusts EDD by -21 days (twins) or -28 (triplets). These are statistical averages. Your MFM specialist determines delivery timing.
Chapter 6: Risk Factors — AMA and Preterm History
AMA (35+) triggers additional screening and monitoring. Preterm history is the strongest predictor of recurrence. ACOG recommends serial cervical length measurements starting at 16 weeks.
Chapter 7: Prenatal Visit Schedule
ACOG schedule: every 4 weeks until 28w, every 2 weeks 28–36w, weekly 36w+. Key milestones: 8w initial visit, 12w NT scan, 20w anatomy scan, 24–28w glucose test, 36w GBS culture.
Chapter 8: Accuracy of Due Dates
Only 4–5% of babies are born on their exact EDD. 80% are born between 37–42 weeks. A "due date" is better understood as a "due window."