# Pregnancy due date calculator

> Pregnancy due date from the last period, conception date or IVF transfer, with today's gestational age, trimester and milestones (ACOG).

Interactive version: https://www.calcopenly.com/health/pregnancy-due-date-calculator
Subject: Health and fitness calculators

The due date is 280 days (40 weeks) after the first day of the last menstrual period, moved one day later for each day a usual cycle runs past 28. From a known conception date it is 266 days later, and after IVF it is 261 days from a day-5 embryo transfer or 263 days from a day-3 transfer, the counts in ACOG Committee Opinion 700. Gestational age, trimester and milestones are counted from the same start.

People use it to date a pregnancy before the first scan and to plan around milestones. A last period that began on 1 October 2025, with a 28-day cycle, gives a due date of 8 July 2026; on 15 January 2026 that pregnancy is 15 weeks 1 day along, in the second trimester.

The date is an estimate for planning. ACOG regards a first-trimester ultrasound as the most accurate way to set or confirm gestational age, and a clinician may change the due date after a scan.

## Naegele's rule and the 280-day count

The German obstetrician Franz Naegele (1778–1851) gave his name to the rule that a pregnancy lasts nine months and seven days from the first day of the last period, [Lawson's 2021 review](https://pubmed.ncbi.nlm.nih.gov/33079400/) notes. In its usual form: take the first day of the last period, add seven days, subtract three months and add one year. ACOG's convention is a plain count of 280 days, and this calculator follows it.

The two methods can disagree by a few days because calendar months are not all the same length. For a last period starting on 3 March 2026:

1. Naegele: 3 March + 7 days = 10 March; minus three months = 10 December 2025; plus one year = 10 December 2026.
2. 280-day count: 3 March 2026 + 280 days = 8 December 2026.

Naegele's rule adds nine calendar months and a week, and the length of those nine months depends on which months they are. From 10 March to 10 December they cover March to November, five months of 31 days and no February, 275 days in all, so the rule lands 282 days after the period. From 8 January to 8 October the nine months include a 28-day February and come to 273 days, so the two methods agree. The table shows the gap for a period starting on the 1st of each month in 2026:

| First day of last period | 280-day due date | Naegele's rule | Naegele later by |
|---|---|---|---|
| 1 Jan 2026 | 8 Oct 2026 | 8 Oct 2026 | 0 days |
| 1 Feb 2026 | 8 Nov 2026 | 8 Nov 2026 | 0 days |
| 1 Mar 2026 | 6 Dec 2026 | 8 Dec 2026 | 2 days |
| 1 Apr 2026 | 6 Jan 2027 | 8 Jan 2027 | 2 days |
| 1 May 2026 | 5 Feb 2027 | 8 Feb 2027 | 3 days |
| 1 Jun 2026 | 8 Mar 2027 | 8 Mar 2027 | 0 days |
| 1 Jul 2026 | 7 Apr 2027 | 8 Apr 2027 | 1 day |
| 1 Aug 2026 | 8 May 2027 | 8 May 2027 | 0 days |
| 1 Sep 2026 | 8 Jun 2027 | 8 Jun 2027 | 0 days |
| 1 Oct 2026 | 8 Jul 2027 | 8 Jul 2027 | 0 days |
| 1 Nov 2026 | 8 Aug 2027 | 8 Aug 2027 | 0 days |
| 1 Dec 2026 | 7 Sep 2027 | 8 Sep 2027 | 1 day |

To check a date by hand, the [add or subtract days calculator](/datetime/add-subtract-days) will add 280 days to any date.

## Dating by period, conception, IVF or ultrasound

[ACOG Committee Opinion 700](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date), written with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine and reaffirmed in 2025, ranks the methods.

- **Last period.** The 280-day count assumes a 28-day cycle with ovulation on day 14. The opinion notes that only about half of women recall the date of their last period accurately. In one trial it cites, 40% of women randomised to a first-trimester scan had their due date moved because the scan and the period date differed by more than 5 days.
- **Conception date.** The due date is 266 days after conception, the same point as 280 days from a period two weeks earlier. A conception date entered in the calculator is treated as the day of ovulation and fertilisation.
- **IVF.** ACOG says a pregnancy from assisted reproduction should be dated from the embryo's age and the transfer date, not from a period.
- **Ultrasound.** A first-trimester scan measuring crown–rump length, up to 13 weeks 6 days, is the most accurate method, to within 5 to 7 days. Later scans are less precise, so ACOG advises changing the due date only when a scan disagrees with the period date by more than a threshold that widens with gestational age:

| Gestational age at the scan | Scan accuracy | Change the due date if the difference exceeds |
|---|---|---|
| Up to 8 weeks 6 days | ±5 to 7 days | 5 days |
| 9w0d to 13w6d | ±5 to 7 days | 7 days |
| 14w0d to 15w6d | ±7 to 10 days | 7 days |
| 16w0d to 21w6d | ±7 to 10 days | 10 days |
| 22w0d to 27w6d | ±10 to 14 days | 14 days |
| 28 weeks and later | ±21 to 30 days | 21 days |

Once the due date is set from the period date or the first reliable scan, ACOG advises changing it only in rare circumstances. The calculator has no scan input: if your clinician has set a date from a scan, use that date.

## Trimester and milestone dates: a worked example

For a last period starting on 3 March 2026 with a 28-day cycle, the milestones fall on these dates. Each is the period date plus the number of days in the gestational age.

| Milestone | Gestational age | Date |
|---|---|---|
| Estimated conception | 2w 0d | 17 Mar 2026 |
| Last day of first trimester | 13w 6d | 8 Jun 2026 |
| Second trimester begins | 14w 0d | 9 Jun 2026 |
| Anatomy scan window | 18w 0d to 22w 6d | 7 Jul to 10 Aug 2026 |
| Third trimester begins | 28w 0d | 15 Sep 2026 |
| Early term | 37w 0d | 17 Nov 2026 |
| Full term | 39w 0d | 1 Dec 2026 |
| Due date | 40w 0d | 8 Dec 2026 |
| Late term | 41w 0d | 15 Dec 2026 |
| Post-term | 42w 0d | 22 Dec 2026 |

The scan window follows [ACOG's patient guidance](https://www.acog.org/womens-health/faqs/ultrasound-exams) that the standard ultrasound exam is usually done at 18 to 22 weeks. A conception date of 17 March 2026, or a day-5 IVF transfer on 22 March 2026, gives the same 8 December due date. With a 35-day cycle instead of 28, every date moves 7 days later and the due date becomes 15 December.

Gestational age is counted in completed weeks and days. On 26 September 2026 the calculator shows this pregnancy at 29 weeks 4 days: 207 days since 3 March, in the third trimester, 73.9% of the way to 280 days, with 73 days to go. A reading of 29 weeks 4 days means the 30th week is under way. With the 35-day cycle the calculator counts from 10 March, 7 days after the period started, so the same day reads 28 weeks 4 days with 80 days to go.

## How often babies arrive on the due date

Few do. [Bergsjø and colleagues](https://pubmed.ncbi.nlm.nih.gov/2220340/) analysed 427,581 singleton births in Sweden from 1976 to 1980. Among women with reliable period dates, the median time from the last period to a vaginal birth was 282 days, two days past the due date, with a standard deviation of about 13 days, and 10% of those women gave birth after day 294, when post-term begins. If births followed a normal curve with that spread, about 3% would fall on any single day at its peak.

Dating method changes the odds of arriving close to the date. In [34,249 pregnancies from the East Midlands](https://pubmed.ncbi.nlm.nih.gov/8572021/), birth came within 7 days of the due date in 49.5% of cases when the period date was used and 55.2% when ultrasound was used; within 10 days, 64.1% and 70.3%.

First pregnancies run longer. [Smith (2001)](https://pubmed.ncbi.nlm.nih.gov/11425837/) studied 1,514 women whose period date and early scan agreed to within a day, looking at births that were not planned inductions or caesareans: the median was 284 days for first-time mothers and 282 days for others.

## Mistakes that shift the date

- **Using the day of a positive test.** The count starts on the first day of the last period, about two weeks before conception in a 28-day cycle, not on the day a test turned positive.
- **Adjusting for an irregular cycle.** The cycle-length field assumes the extra days all fall before ovulation. When cycle length varies from month to month there is no single right number to enter, and ACOG advises dating by an early scan when the period date is uncertain.
- **Entering the date of sex as the conception date.** In [Wilcox and colleagues' study](https://pubmed.ncbi.nlm.nih.gov/7477165/) of 221 women, pregnancies came from intercourse up to five days before ovulation, so conception can follow sex by several days.

The calculator refuses a start date after today, and any date that would put the pregnancy past 44 weeks, with a prompt to check the year. For cycle tracking before pregnancy, the [ovulation calculator](/health/ovulation-calculator) estimates fertile days from the same cycle-length input.

## Inputs

- **Calculate from** (options: First day of last period, Conception date, IVF embryo transfer date)
- **First day of last period**
- **Usual cycle length**: The 280-day rule assumes a 28-day cycle; each extra day moves the due date one day later.
- **Conception date**
- **Transfer date**
- **Embryo age at transfer** (options: Day 3, Day 5 (blastocyst))

## Results

- Estimated due date — main result
- Gestational age today
- Trimester
- Days until the due date
- Estimated conception
- Full term from (39 weeks)
- Progress through 40 weeks

## Formula

$$
\text{EDD} = \text{LMP} + 280\ \text{d} + (\text{cycle} - 28)\ \text{d} = \text{conception} + 266\ \text{d} = \text{transfer} + (266 - \text{embryo age})\ \text{d}
$$

## Worked examples

### LMP 1 Oct 2025, 28-day cycle

- Calculate from: First day of last period
- First day of last period: 2025-10-01
- Usual cycle length: 28 days
- **Estimated due date: 2026-07-08**
- **Gestational age today: 15 weeks 1 day**
- **Trimester: Second**
- **Days until the due date: 174**
- Checked against: Python 3.8 datetime: date(2025,10,1) + timedelta(280); GA = date(2026,1,15) − LMP = 106 d (ACOG CO 700 280-day rule)

### LMP 1 Nov 2025, 32-day cycle

- Calculate from: First day of last period
- First day of last period: 2025-11-01
- Usual cycle length: 32 days
- **Estimated due date: 2026-08-12**
- **Gestational age today: 10 weeks 1 day**
- **Trimester: First**
- Checked against: Python 3.8 datetime: LMP + 280 + (32 − 28) days; GA measured from the adjusted LMP (due − 280 d)

### IVF day-5 transfer on 1 Dec 2025

- Calculate from: IVF embryo transfer date
- Transfer date: 2025-12-01
- Embryo age at transfer: Day 5 (blastocyst)
- **Estimated due date: 2026-08-19**
- **Gestational age today: 9 weeks 1 day**
- **Estimated conception: 2025-11-26**
- Checked against: ACOG CO 700: day-5 transfer + 261 days; GA at transfer = 2w5d. Python 3.8 datetime check

### IVF day-3 transfer on 1 Dec 2025

- Calculate from: IVF embryo transfer date
- Transfer date: 2025-12-01
- Embryo age at transfer: Day 3
- **Estimated due date: 2026-08-21**
- **Gestational age today: 8 weeks 6 days**
- Checked against: ACOG CO 700: day-3 transfer + 263 days; GA at transfer = 2w3d. Python 3.8 datetime check

### Conception 1 Jun 2025

- Calculate from: Conception date
- Conception date: 2025-06-01
- **Estimated due date: 2026-02-22**
- **Gestational age today: 34 weeks 4 days**
- **Trimester: Third**
- **Days until the due date: 38**
- Checked against: Python 3.8 datetime: conception + 266 days; GA = today − (conception − 14 d)

### Boundary: exactly 14 weeks 0 days

- Calculate from: First day of last period
- First day of last period: 2025-10-09
- Usual cycle length: 28 days
- **Gestational age today: 14 weeks 0 days**
- **Trimester: Second**
- **Estimated due date: 2026-07-16**
- Checked against: ACOG trimester boundary 14w0d; Python 3.8 datetime: 2026-01-15 − 2025-10-09 = 98 days

## Questions

### How is a due date calculated?

Add 280 days (40 weeks) to the first day of the last menstrual period. This assumes a 28-day cycle with ovulation around day 14; for a longer or shorter cycle the date shifts by the difference, so a 32-day cycle adds 4 days. Naegele's rule, add one year, subtract three months and add seven days, gives the same or a nearby date: a period starting on 1 October 2025 gives 8 July 2026 either way.

### Why might a baby not arrive on the due date?

Because pregnancy length varies naturally, even when conception is dated exactly. In a study of 125 natural pregnancies with ovulation identified from hormone tests (Jukic and colleagues, 2013), the median time from ovulation to birth was 268 days, and lengths spread over 37 days after excluding preterm births. Dating from the last period adds more variation, because the day of ovulation differs between cycles.

### When does each trimester start?

Counted from the first day of the last period, 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. For a period that began on 1 October 2025 (due 8 July 2026), the second trimester starts on 7 January 2026 and the third on 15 April 2026.

### What do early term, full term and late term mean?

ACOG Committee Opinion 579 (2013), endorsed by the Society for Maternal-Fetal Medicine, defines early term as 37 weeks 0 days to 38 weeks 6 days, full term as 39 weeks 0 days to 40 weeks 6 days, late term as 41 weeks 0 days to 41 weeks 6 days, and post-term as 42 weeks 0 days or later. The labels replaced a single 'term' category because newborn outcomes differ across that five-week span.

### How is the due date worked out after IVF?

From the embryo transfer date and the embryo's age at transfer. ACOG Committee Opinion 700 sets the due date 263 days after a day-3 embryo transfer and 261 days after a day-5 blastocyst transfer, which is 266 days minus the embryo's age. A day-5 transfer on 1 December 2025 therefore gives a due date of 19 August 2026.

### How accurate is the pregnancy due date calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 7 worked examples whose answers come from independent sources; for example, “LMP 1 Oct 2025, 28-day cycle” is checked against Python 3.8 datetime: date(2025,10,1) + timedelta(280); GA = date(2026,1,15) − LMP = 106 d (ACOG CO 700 280-day rule).

### Where does the method come from?

ACOG Committee Opinion No. 700 — Methods for estimating the due date (2017); ACOG — How your fetus grows during pregnancy (trimester boundaries); ACOG Committee Opinion No. 579 — Definition of term pregnancy (2013).

## Sources

- [ACOG Committee Opinion No. 700 — Methods for estimating the due date (2017)](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date)
- [ACOG — How your fetus grows during pregnancy (trimester boundaries)](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy)
- ACOG Committee Opinion No. 579 — Definition of term pregnancy (2013)

_Note: medical information, not professional advice._