# What pregnancy week am I in?

Works out which week of pregnancy you are in on a date, with weeks and days, the trimester and the dates that week starts and ends, from the due date, last period, an ultrasound or the conception date.

- Page: https://www.acalculator.org/health/pregnancy-week-calculator
- JSON spec: https://www.acalculator.org/health/pregnancy-week-calculator.json
- Version: c55d9360810b

## Default answer

Example with the default inputs (Based on Due date, Your due date March 10, 2027, Work out on September 30, 2026) on the example date Wednesday, September 30, 2026: On September 30, 2026 you are 17 weeks pregnant, in pregnancy week 18 (Second trimester).

## 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 |
| --- | --- | --- |
| week | Pregnancy week | The week of pregnancy the date falls in: completed weeks + 1 (17 weeks 3 days is week 18). |
| ga | Weeks pregnant | Completed weeks and days of pregnancy on the date, counted from the start. |
| trimester | Trimester | The trimester on the date (ACOG), or a note when the date is outside the pregnancy. |
| weekStart | This week started | The first day of the pregnancy week the date falls in. |
| weekEnd | This week ends | The last day of the pregnancy week the date falls in. |
| due | Due date | The estimated due date: 280 days (40 weeks) after the start. |
| daysLeft | Days until the due date | Days from the date to the due date; left out after the due date. |
| start | Pregnancy counted from | Day 0 of the count: the first day of the last period, adjusted for cycle length. |

## Method

Days pregnant = date − start; pregnancy week = ⌊days ÷ 7⌋ + 1, where the start is the due date − 280 days, the last period + (cycle − 28) days, the ultrasound date − the 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). Week 1 is days 0 to 6.
- Trimesters: first to 13 weeks 6 days, second from 14 weeks 0 days, third from 28 weeks 0 days.
- Before the start, or more than 42 weeks 6 days after it, the week is left out and a note says so.
- An estimate, not medical advice: your provider sets your dates, often from an early ultrasound.

## Worked examples

1. method = due, due = 2027-03-01, on = 2026-09-29 gives start = 2026-05-25, ga = {"weeks":18,"days":1}, week = 19, weekStart = 2026-09-28, weekEnd = 2026-10-04, trimester = Second trimester, daysLeft = 153. Source: ACOG Committee Opinion 700 (due date = start + 280 days).
2. method = lmp, lmp = 2026-08-01, cycle = 28, on = 2026-09-29 gives ga = {"weeks":8,"days":3}, week = 9, trimester = First trimester, due = 2027-05-08. Source: ACOG Committee Opinion 700 (last period + 280 days).
3. method = lmp, lmp = 2026-01-05, cycle = 28, on = 2026-10-12 gives ga = {"weeks":40,"days":0}, week = 41, daysLeft = 0, trimester = Third trimester. Source: ACOG Committee Opinion 700: day 280 is the due date.

## FAQ

### What pregnancy week am I in?

Count the whole weeks since the first day of your last period and add 1. At 17 weeks and 3 days you have finished 17 weeks and are in your 18th week, week 18. Many apps and books say "17 weeks pregnant" for the same day, so check whether a guide counts completed weeks or the week you are in.

### How do I work out my pregnancy week from my due date?

Count back 280 days (40 weeks) from the due date to find the start, then count the days from the start to today. With a due date of March 1, 2027, on September 29, 2026 it is 127 days since May 25, 2026: 18 weeks 1 day, so week 19.

### When does each trimester start?

The second trimester starts at 14 weeks 0 days (the start of week 15) and the third at 28 weeks 0 days (the start of week 29). The first trimester runs to 13 weeks 6 days.

### Why do pregnancy weeks start before conception?

Doctors count from the first day of the last period, because most people know that date and not the day of conception. Conception is usually about 2 weeks later, so in week 1 and most of week 2 you are not yet pregnant.

### What happens at 40 weeks?

The due date is 40 weeks 0 days, the first day of week 41. ACOG calls a birth from 37 weeks 0 days to 38 weeks 6 days early term, from 39 weeks 0 days to 40 weeks 6 days full term, in week 42 late term, and from 42 weeks 0 days post-term.

### Which date should I use?

Use the due date your provider gave you, which is often set by an early 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