Pregnancy dating methods and when clinical dating takes priority
Estimate a pregnancy due date using last menstrual period, known conception, embryo-transfer, or ultrasound dating arithmetic.
This page provides an equation-based estimate for education and planning. It does not diagnose a condition, measure an individual response, or replace professional care.
How to use this health calculator
- Choose appropriate inputs: Enter measurements using the units and ranges shown.
- Review the assumptions: Confirm that the model applies before interpreting the result.
- Calculate and interpret: Read the result together with its limitations and cautions.
- Use professional guidance when needed: Do not use a general calculator to change medication, treatment, or a clinician-directed plan.
Formula and variables
These rules translate the selected dating information to a 40-week gestational scale. The result is provisional until maternity care establishes the best obstetric estimate.
LMP + 280 days; conception + 266 days; day-5 transfer + 261 days; day-3 transfer + 263 days; ultrasound date + [280 − gestational days at scan]- LMP — Last menstrual period
- First day of the last menstrual period.
- GA — Gestational age at scan
- Completed weeks and days assigned on the ultrasound date.
Worked example: LMP dating
The first day of the last menstrual period is January 1, 2026.
- LMP
- January 1, 2026
- Add 280 days to the first day of the LMP.
Result: The arithmetic estimated due date is October 8, 2026.
This assumes a 28-day cycle with ovulation near day 14; irregular cycles or uncertain dates reduce reliability.
Understanding your results
Equation result
The numerical output follows the displayed formula and entered assumptions.
Personal interpretation
Individual biology, measurement error, environment, health conditions and medicines can change real-world outcomes.
Assumptions
- The entered calendar date is correct.
- LMP dating assumes a regular 28-day cycle and ovulation around day 14.
- The IVF embryo age and transfer date are known accurately.
- Ultrasound weeks and days are the clinician-assigned gestational age on that scan date.
Limitations
- Only the maternity-care team can establish or revise the best obstetric estimated due date.
- First-trimester ultrasound is generally more accurate than later ultrasound for establishing gestational age.
- The calculation does not assess fetal growth, pregnancy viability or timing of delivery.
- Most births do not occur exactly on the estimated due date.
Common mistakes
- Entering the last day rather than first day of the LMP.
- Using egg retrieval instead of embryo transfer date.
- Treating conception date as intercourse date when fertilization timing is unknown.
- Repeatedly changing the due date from later scans without clinical review.
Practical use cases
Educational estimate
Explore how the inputs affect the equation result.
Consistent tracking
Compare estimates only when methods, units and conditions are consistent.
Planning and decision guide
Treat cautions as part of the result
Use the documented due date from maternity care for appointments, testing and clinical decisions.
Seek urgent maternity care for concerning symptoms such as heavy bleeding, severe pain, fluid leakage, reduced fetal movement when applicable, or signs of labor according to clinical instructions.
Escalate symptoms or clinical questions
Seek qualified medical advice for symptoms, diagnosed conditions, medicines, pregnancy, or individualized treatment decisions.
Frequently asked questions
Why does LMP add 280 days?
Obstetric gestational age convention counts 40 weeks from the first day of the last menstrual period.
How is a day-5 IVF transfer calculated?
ACOG describes adding 261 days to the embryo replacement date for a day-5 embryo.
Should a later ultrasound change my due date?
Usually not automatically. Redating depends on gestational age, discrepancy and clinical guidance.
Sources and review
- Methods for Estimating the Due Date — American College of Obstetricians and Gynecologists. Accessed 2026-08-03.
- When Pregnancy Goes Past Your Due Date — American College of Obstetricians and Gynecologists. Accessed 2026-08-03.
Reviewed 2026-08-03.