# Pregnancy Due Date Calculator

> Estimate a pregnancy due date from last menstrual period or conception date, with trimester dates.

Page: https://freecalculator.tech/tools/pregnancy-due-date-calculator

## How do I estimate a pregnancy due date?

A due date is commonly estimated as 280 days from the first day of the last menstrual period, or 266 days from conception.

## Formula

Due date from last menstrual period uses Naegele's rule: due date = LMP + 280 days, adjusted by cycle length − 28 days. Due date from conception uses conception date + 266 days.

- LMP method: estimated due date = first day of last menstrual period + 280 days + (average cycle length − 28 days).
- Conception method: estimated due date = conception date + 266 days, which is equivalent to about 38 weeks from conception.
- Trimester dates are counted from the adjusted pregnancy start: first trimester through 13 weeks 6 days, second trimester from 14 weeks through 27 weeks 6 days, and third trimester from 28 weeks to the due date.
- Due date from last menstrual period uses Naegele's rule: due date = LMP + 280 days, adjusted by cycle length − 28 days. Due date from conception uses conception date + 266 days.
- Inputs used: Date type, First day of last menstrual period, Average cycle length, Conception date.
- Edge handling: First day of last menstrual period is required.; First day of last menstrual period must use a valid calendar date.; Average cycle length is required.; Average cycle length must be at least 21.; Average cycle length must be no more than 45..

## Inputs

- Date type
- First day of last menstrual period
- Average cycle length
- Conception date

## Worked example: LMP on Jan 1, 2026 with a 28-day cycle

Adding 280 days gives an estimated due date of Oct 8, 2026, with the third trimester starting around Jul 16, 2026.
1. Start with the first day of the last menstrual period: Jan 1, 2026.
2. Use a 28-day cycle, so the cycle adjustment is 28 − 28 = 0 days.
3. Add 280 days: Jan 1, 2026 + 280 days = Oct 8, 2026.
4. Count trimesters from the adjusted start date: first through day 97, second through day 195, and third from day 196 to the due date.

## Common questions

### How do I estimate a pregnancy due date?

A due date is commonly estimated as 280 days from the first day of the last menstrual period, or 266 days from conception. Check the formula, example, and limitations on this page before using the result for a real health & fitness decision.

### Is the due date exact?

No. A due date is an estimate, and many pregnancies deliver before or after that date. Ultrasound dating and clinical history may change the date used by a care team.

### Why does cycle length change the LMP result?

Naegele's rule assumes a 28-day cycle with ovulation around day 14. Longer or shorter average cycles shift the estimated ovulation date, so this calculator adjusts the due date by the difference from 28 days.

### Which method should I choose?

Use last menstrual period when you know the first day of the last period and conception date when ovulation, insemination, or fertility-treatment timing is known. If a clinician has given you a due date, use that clinical date rather than recalculating it here.

### How do I use the Pregnancy Due Date Calculator?

Enter the required values for Date type, First day of last menstrual period, Average cycle length, Conception date. The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.

### What formula does the Pregnancy Due Date Calculator use?

Due date from last menstrual period uses Naegele's rule: due date = LMP + 280 days, adjusted by cycle length − 28 days. Due date from conception uses conception date + 266 days. The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.

## Limitations and assumptions

- This estimate does not replace ultrasound dating, fertility-treatment dating, or clinician guidance.
- Cycle-length adjustment is a simple arithmetic correction and cannot confirm actual ovulation or implantation timing.
- The trimester breakdown is approximate and may differ from how a clinician records pregnancy weeks in a chart.
- Estimate for informational purposes only; not medical advice, diagnosis or treatment.
- Inputs outside the supported range are rejected rather than forced into a result.

## Cite this page

- APA: Free Calculator. (n.d.). Pregnancy Due Date Calculator. https://freecalculator.tech/tools/pregnancy-due-date-calculator
- MLA: "Pregnancy Due Date Calculator." Free Calculator, https://freecalculator.tech/tools/pregnancy-due-date-calculator.
- Link: Pregnancy Due Date Calculator — Free Calculator, https://freecalculator.tech/tools/pregnancy-due-date-calculator