# How far along is my pregnancy?

Works out how many weeks and days pregnant you are on a date, the trimester, the due date and milestone dates, from the last period, the due date, an ultrasound or the conception date.

- Page: https://www.acalculator.org/health/pregnancy-calculator
- JSON spec: https://www.acalculator.org/health/pregnancy-calculator.json
- Version: 89983ae0475a

## Default answer

Example with the default inputs (Based on Last period, First day of your last period July 7, 2026, Cycle length (days) 28, Work out on September 29, 2026) on the example date Tuesday, September 29, 2026: On September 29, 2026 you are 12 weeks pregnant (First trimester). Estimated due date: April 13, 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. |
| due | Your due date | The estimated due date you were given. |
| on | Work out on | The date to work out how far along the pregnancy is. Today, unless you change it. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| ga | How far along | Weeks and days of pregnancy on the work-out date, counted from the start. |
| stage | Stage | The trimester on the work-out date, or a note when the date is outside the pregnancy. |
| due | Estimated due date | The estimated due date: 280 days (40 weeks) after the start. |
| daysLeft | Days until the due date | Days from the work-out date to the due date. |
| done | Share of 40 weeks | How much of the 280 days to the due date has passed. |
| start | Pregnancy counted from | Day 0 of the count: the first day of the last period, adjusted for cycle length. |
| conception | Likely conception date | About 14 days after the start, or the conception date you gave. |
| second | Second trimester starts | The first day of the second trimester: 14 weeks 0 days. |
| third | Third trimester starts | The first day of the third trimester: 28 weeks 0 days. |
| fullTerm | Full term from | The start of full term: 39 weeks 0 days (ACOG). |

## Method

Weeks pregnant = (date − start) ÷ 7, where the start is the first day of the last period + (cycle length − 28) days, the due date − 280 days, the ultrasound date − the gestational age it gave, or the conception date − 14 days.

## 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 from 14 weeks 0 days, third from 28 weeks 0 days. Full term starts at 39 weeks 0 days.
- Before the start, or more than 44 weeks after it, how far along is left out and a note says so.
- An estimate, not medical advice: a first-trimester ultrasound is the most accurate way to date a pregnancy.

## Worked examples

1. method = lmp, lmp = 2026-06-01, cycle = 28, on = 2026-09-27 gives ga = {"weeks":16,"days":6}, stage = Second trimester, due = 2027-03-08, daysLeft = 162, done = 42.142857%, second = 2026-09-07, third = 2026-12-14, fullTerm = 2027-03-01. Source: ACOG Committee Opinion 700 (LMP + 280 days) and trimester limits; hand calculation in content.mdx.
2. method = due, due = 2026-12-25, on = 2026-09-27 gives start = 2026-03-20, ga = {"weeks":27,"days":2}, stage = Second trimester, daysLeft = 89. Source: ACOG CO 700 (due date = start + 280 days); hand calculation in content.mdx.
3. method = conception, conception = 2026-02-14, on = 2026-09-27 gives ga = {"weeks":34,"days":1}, stage = Third trimester, due = 2026-11-07, daysLeft = 41. Source: ACOG CO 700 (conception about day 14); hand calculation in content.mdx.
4. method = ultrasound, scan = 2026-07-01, scanWeeks = 8, scanDays = 2, on = 2026-09-27 gives start = 2026-05-04, ga = {"weeks":20,"days":6}, due = 2027-02-08. Source: ACOG CO 700 (dating by ultrasound); hand calculation in content.mdx.

## FAQ

### How do I work out how many weeks pregnant I am?

Count the days from the first day of your last period to today, then divide by 7. The whole number is the weeks and the remainder is the days. For example, from June 1 to September 27, 2026 is 118 days, which is 16 weeks 6 days.

### Why are pregnancy weeks counted from my last period?

The first day of the last period is a date most people know, while the day of conception usually is not. Counting from it adds about 2 weeks before conception, so a pregnancy is 40 weeks (280 days) long on this count.

### I know my due date. How far along am I?

Choose "Due date". The calculator counts back 280 days to find the start, then counts forward to today. With a due date of December 25, 2026, on September 27, 2026 you are 27 weeks 2 days pregnant.

### When does each trimester start?

The second trimester starts at 14 weeks 0 days and the third at 28 weeks 0 days. The first trimester runs to 13 weeks 6 days. The calculator shows the dates for your pregnancy.

### What does full term mean?

ACOG calls a birth from 39 weeks 0 days to 40 weeks 6 days full term. From 37 weeks 0 days to 38 weeks 6 days is early term, 41 weeks is late term, and 42 weeks or more is post-term.

### Which date should I use?

Use the due date your provider gave you if you have one, because it may come from an ultrasound. ACOG says a first-trimester ultrasound is the most accurate way to date a pregnancy.

## 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/