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Health & Fitness

Pregnancy Due Date Estimator

Estimate a pregnancy due date from the last menstrual period (adjusted for cycle length), conception date or IVF transfer date.

Your inputs

Results

Estimated due date

Thursday, October 8, 2026

Due date (ISO)
2026-10-08
Second trimester begins (week 14)
Thursday, April 2, 2026
Third trimester begins (week 28)
Thursday, July 9, 2026

≈Estimate based on the stated model and assumptions

Notes for this result

  • Only about 4–5 % of babies arrive on the estimated due date. An early ultrasound dating scan is more accurate than date-based estimates. This tool does not replace prenatal care.

Inputs are never added to links for financial, salary or health calculators.

What this calculator does

Due dates are conventionally counted as 280 days (40 weeks) from the first day of the last menstrual period, adjusted for longer or shorter cycles.

Formula and method

LMP: due = LMP + 280 days + (cycle length − 28) Conception: + 266 days; IVF day-5 transfer: + 261 days; day-3: + 263 days

Worked example

Calculated by this tool with its default inputs:

Inputs

  • Based on: First day of last menstrual period
  • Date: 2026-01-01
  • Average cycle length: 28 days

Results

  • Estimated due date: Thursday, October 8, 2026
  • Due date (ISO): 2026-10-08
  • Second trimester begins (week 14): Thursday, April 2, 2026
  • Third trimester begins (week 28): Thursday, July 9, 2026

How to read the results

Only a small share of babies are born on the due date itself; most arrive within a couple of weeks either side.

Assumptions and limitations

  • An early ultrasound gives more accurate dating.
  • This does not replace prenatal care.

Health results are general estimates for adults and do not replace medical advice, diagnosis or treatment. Speak to a qualified health professional about your personal situation, especially during pregnancy or if you have a medical condition.

Questions

What is Naegele’s rule?

The traditional rule of adding 280 days (or one year, minus three months, plus seven days) to the last menstrual period.

Methodology and verification

Result type
Estimate based on the stated model and assumptions
Formula version
1.0
Automated verification
5 reference and validation test cases, checked against independently computed values. The site cannot be built if any of them fail.
Editorial review
Awaiting human editorial review. The explanation was drafted with AI assistance; the formula and examples are verified by the automated tests above.

Privacy

Everything you enter is calculated in this browser and is not sent to Numerith or anyone else. Because this calculator can involve personal financial or health details, your inputs are never added to share links or saved to history. Privacy policy