Pregnancy Due Date Calculator
LMP, conception, IVF transfer (any day, fresh or frozen), IUI, ultrasound CRL — all in one tool, with ACOG-compliant redating when LMP and ultrasound disagree.
How the math works
Skip the math — try the calculator aboveEvery dating method ultimately maps to one estimated date of delivery (EDD), 40 weeks from the last menstrual period (LMP) or equivalent reference point. Different methods come from different starting points:
- LMP (Naegele's rule): EDD = LMP + 280 days. Assumes a 28-day cycle with ovulation on day 14. We adjust for cycle length: a 32-day cycle pushes the EDD 4 days later; a 21-day cycle pulls it 7 days earlier.
- Conception date: EDD = conception + 266 days. Used when conception is known (rare outside IVF/IUI).
- IVF transfer: EDD = transfer date + (266 − embryo age in days). Day-3 transfer adds 263 days; Day-5 adds 261; Day-6 adds 260. Fresh and frozen produce identical EDDs — the embryo's biological age is what matters, not whether it was frozen.
- IUI date: EDD = IUI date + 266 days. Sperm typically meets egg within 24 hours of insemination, so IUI is treated as fertilization day.
- Ultrasound CRL (Robinson formula): GA in days = 8.052 × √CRL + 23.73. Accurate for crown-rump-length 5-84mm (~5w-13w6d). EDD = ultrasound date + (280 − GA).
ACOG redating policy (Committee Opinion 700)
When LMP and ultrasound disagree, ACOG sets specific thresholds for when the ultrasound-derived EDD should replace the LMP-derived EDD. The earlier in pregnancy, the tighter the threshold:
| Ultrasound GA | Discrepancy threshold | If exceeded |
|---|---|---|
| < 9w0d | > 5 days | Use ultrasound |
| 9w0d – 13w6d | > 7 days | Use ultrasound |
| 14w0d – 15w6d | > 7 days | Use ultrasound |
| 16w0d – 21w6d | > 10 days | Use ultrasound |
| 22w0d – 27w6d | > 14 days | Use ultrasound |
| ≥ 28w0d | Ultrasound too imprecise to override LMP | Keep LMP |
Our calculator's LMP + ultrasound mode applies these thresholds automatically. We show you both candidate due dates, the discrepancy in days, the ACOG threshold for the gestational age of the ultrasound, and the recommended EDD with rationale.
Sources
- ACOG Committee Opinion 700: Methods for Estimating the Due Date. American College of Obstetricians and Gynecologists, May 2017 (reaffirmed). acog.org
- Robinson HP, Fleming JE. A critical evaluation of sonar 'crown-rump length' measurements. Br J Obstet Gynaecol. 1975;82(9):702-710.
- Hadlock FP, Harrist RB, Martinez-Poyer J. In utero analysis of fetal growth: a sonographic weight standard. Radiology. 1991;181(1):129-133. (50th-percentile fetal weight reference used in our weekly development notes.)
Frequently asked questions
How accurate is a due date calculator?
A due date is an estimate, not a deadline. Only about 5% of births occur on the exact due date, and most occur within the 38-42 week window. The most accurate method is a first-trimester ultrasound (crown-rump length, ideally before 13w6d) — accurate to within ±5 days. Last menstrual period (LMP) dating assumes a regular 28-day cycle and ovulation on day 14; for cycles that differ, the due date can be off by a week or more. IVF and IUI dates are precise because conception timing is known.
What's the difference between fresh and frozen embryo transfer for due date math?
None — the due date math is identical. What matters is the embryo's biological age at transfer (Day-3 vs Day-5 vs Day-6), not whether it was frozen beforehand. A Day-5 fresh transfer and a Day-5 frozen transfer (FET) produce the same due date when calculated from the transfer date. Many calculators get this wrong by treating fresh and frozen as different methods; they're not.
What is ACOG's policy when LMP and ultrasound disagree?
ACOG Committee Opinion 700 ("Methods for Estimating the Due Date") sets discrepancy thresholds: under 9 weeks, a >5-day discrepancy triggers redating to the ultrasound; from 9-13w6d and 14-15w6d, the threshold is >7 days; from 16-21w6d, >10 days; from 22-27w6d, >14 days. After 28 weeks, ultrasound dating is too imprecise to override LMP. Our calculator's LMP + ultrasound mode applies these thresholds automatically and shows you which date ACOG recommends and why.
Why does cycle length matter for LMP-based calculations?
Naegele's rule (LMP + 280 days) assumes a 28-day cycle with ovulation on cycle day 14. If your cycle is longer or shorter, ovulation shifts and the due date shifts with it. A 32-day cycle pushes ovulation later by 4 days, so the due date moves 4 days later. A 21-day cycle pulls ovulation forward by 7 days. Most calculators don't adjust for cycle length, which can produce a due date a week or more off. Ours does.
What's the Robinson formula and why does it matter for ultrasound dating?
The Robinson formula (Robinson & Fleming, 1975) converts crown-rump length (CRL, measured in millimeters) into gestational age: GA in days = 8.052 × √CRL + 23.73. It's accurate to ±5 days for CRL between 5mm and 84mm (roughly 5-13w6d gestation). Beyond 84mm, second-trimester biometry (head circumference, biparietal diameter, femur length) is more accurate — our calculator currently supports CRL-based dating only, which covers the first-trimester ultrasound window where redating decisions are typically made.
Can I use this calculator after a pregnancy loss?
Yes, and the output is intentionally factual rather than celebratory. There are no automated "your baby is the size of X" updates that arrive after you stop visiting the page. The calculator is one-shot: you enter the dates, see the math, and leave. If you've experienced a loss and need leave-rights information or recovery resources, our pregnancy after loss page covers federal protections (FMLA, PWFA), state reproductive-loss leave laws, and crisis/community resources.
What's the conception date and how is it different from the due date?
The conception date is when sperm fertilized egg — about 14 days after the first day of your last menstrual period in a typical 28-day cycle. The due date is the conception date plus 266 days (38 weeks of fetal development) — equivalent to LMP + 280 days (40 weeks, because LMP-based gestational age starts 2 weeks before actual conception). For IVF, the "conception" is the day the embryo was created in the lab, which is the transfer day minus the embryo age (e.g., a Day-5 transfer was created 5 days earlier).