# When is my due date?

Estimates a pregnancy due date: 280 days after the first day of the last period (adjusted for cycle length), 266 days after conception, or from an ultrasound, with how far along the pregnancy is.

- Page: https://www.acalculator.org/health/due-date-calculator
- JSON spec: https://www.acalculator.org/health/due-date-calculator.json
- Version: b806dfad8a55

## Default answer

Example with the default inputs (Based on Last period, First day of your last period August 4, 2026, Cycle length (days) 28, Work out on September 29, 2026) on the example date Tuesday, September 29, 2026: Your estimated due date is Tuesday, May 11, 2027.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| method | Based on | What you know: the first day of your last period, the conception date, an ultrasound, or the due date. |
| lmp | First day of your last period | The first day of your last menstrual period. |
| cycle | Cycle length (days) | Days from the first day of one period to the first day of the next. 28 if you are not sure. |
| conception | Date of conception | The day you conceived. |
| scan | Ultrasound date | The day of the ultrasound scan. |
| scanWeeks | Weeks at the scan | The gestational age the scan gave: whole weeks. |
| scanDays | And days | The gestational age the scan gave: days past the whole weeks, 0 to 6. |
| on | Work out on | The date to work out how far along the pregnancy is. Today, unless you change it. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| due | Estimated due date | The estimated due date: 280 days (40 weeks) after the start of the pregnancy count. |
| ga | How far along | Weeks and days of pregnancy on the work-out date, counted from the start. |
| trimester | Trimester | The trimester on the work-out date. |
| note | Note | A note when the work-out date is before the start of the pregnancy count or more than 44 weeks after it. |
| daysLeft | Days until the due date | Days from the work-out date to the due date. |
| conception | Likely conception date | About 14 days after the start of the pregnancy count, or the conception date you gave. |
| fullTermFrom | Full term from | The start of full term: 39 weeks 0 days (ACOG). |
| fullTermTo | Full term to | The end of full term: 40 weeks 6 days (ACOG). |
| start | Pregnancy counted from | Day 0 of the pregnancy count: the first day of the last period, adjusted for cycle length. |

## Method

Due date = start + 280 days, where the start is the first day of the last period + (cycle length − 28) days, the conception date − 14 days, or the ultrasound date − the gestational age it gave.

## Assumptions

- A pregnancy is counted from the first day of the last period and lasts 280 days (40 weeks) to the due date (ACOG).
- Ovulation and conception happen 14 days before the next period would start, so a cycle longer or shorter than 28 days moves the dates by the difference.
- Trimesters: first to 13 weeks 6 days, second to 27 weeks 6 days, third after. Full term is 39 weeks 0 days to 40 weeks 6 days.
- How far along is left out before the start or more than 44 weeks after it, and a note says to check the dates.
- An estimate, not medical advice: a first-trimester ultrasound is the most accurate way to date a pregnancy.

## Worked examples

1. method = lmp, lmp = 2026-01-01, cycle = 28, on = 2026-03-15 gives due = 2026-10-08, ga = {"weeks":10,"days":3}, trimester = First trimester, daysLeft = 207, conception = 2026-01-15, fullTermFrom = 2026-10-01, fullTermTo = 2026-10-14. Source: ACOG Committee Opinion 700 (LMP + 280 days); ACOG CO 579 (full term); hand calculation in content.mdx.
2. method = lmp, lmp = 2027-12-01, cycle = 35, on = 2027-12-01 gives start = 2027-12-08, due = 2028-09-13, conception = 2027-12-22. Source: ACOG CO 700 with the cycle-length adjustment in content.mdx; hand calculation in content.mdx.
3. method = conception, conception = 2026-06-10, on = 2026-09-27 gives start = 2026-05-27, due = 2027-03-03, ga = {"weeks":17,"days":4}, trimester = Second trimester. Source: ACOG CO 700 (280 days from the LMP, conception on day 14, so 266 days from conception); hand calculation in content.mdx.
4. method = ultrasound, scan = 2026-08-20, scanWeeks = 12, scanDays = 3, on = 2026-08-20 gives start = 2026-05-25, due = 2027-03-01, ga = {"weeks":12,"days":3}, daysLeft = 193. Source: ACOG CO 700 (dating by ultrasound); hand calculation in content.mdx.

## FAQ

### How is a due date calculated?

A due date is 280 days (40 weeks) after the first day of your last menstrual period. This is Naegele's rule, and it is how the American College of Obstetricians and Gynecologists (ACOG) describes dating from the last period. For example, a last period that started on January 1, 2026 gives a due date of October 8, 2026.

### What if my cycle is not 28 days?

The 280-day rule assumes a 28-day cycle with ovulation on day 14. Ovulation usually happens about 14 days before the next period, so in a longer cycle it happens later. The calculator moves the dates by your cycle length minus 28 days: a 35-day cycle moves the due date 7 days later.

### How do I calculate my due date from the conception date?

Add 266 days (38 weeks) to the conception date. This is the same as counting 280 days from a last period 14 days before conception.

### Why does my ultrasound give a different due date?

ACOG says an ultrasound in the first trimester (up to 13 weeks 6 days) is the most accurate way to date a pregnancy. If your provider changed your due date after a scan, choose "Ultrasound" and enter the scan date and the weeks and days it gave.

### How accurate is a due date?

It is an estimate. Only a small share of babies arrive on the exact day, and most are born in the weeks around it. ACOG calls 39 weeks 0 days to 40 weeks 6 days full term, and the calculator shows those dates too.

### What are the trimesters?

The first trimester runs to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until birth. Weeks are counted from the first day of the last period, so you are about 2 weeks pregnant at conception.

## Sources

- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
- American College of Obstetricians and Gynecologists. Committee Opinion No. 579: Definition of Term Pregnancy. Obstet Gynecol. 2013;122(5):1139-1140. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2013/11/definition-of-term-pregnancy
- Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med. 2019;2:83. https://pmc.ncbi.nlm.nih.gov/articles/PMC6710244/