acalculator

What pregnancy week am I in?

Enter your due date (or the first day of your last period, an ultrasound result or the conception date) to see which pregnancy week you are in today, with weeks and days, the trimester and the dates of this week.

Your numbers

Based on
Use another date
Pregnancy week
18

On September 30, 2026 you are 17 weeks pregnant, in pregnancy week 18 (Second trimester).

Weeks pregnant
17 weeks
Trimester
Second trimester
This week started
September 30, 2026
This week ends
October 6, 2026
Due date
March 10, 2027
Days until the due date
161
Pregnancy counted from
June 3, 2026

Answer for the example date Wednesday, September 30, 2026. It changes to today's date when the page loads.

Pregnancy week: 18. On September 30, 2026 you are 17 weeks pregnant, in pregnancy week 18 (Second trimester).

Where is this week in the pregnancy?

This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use

How to calculate

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.

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).

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.

  • 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.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Based on Due date, Your due date 2027-03-01, Work out on 2026-09-29 gives Pregnancy counted from 2026-05-25, Weeks pregnant 18 weeks, 1 day, Pregnancy week 19, This week started 2026-09-28, This week ends 2026-10-04, Trimester Second trimester, Days until the due date 153.Source: ACOG Committee Opinion 700 (due date = start + 280 days)
  2. Based on Last period, First day of your last period 2026-08-01, Cycle length (days) 28, Work out on 2026-09-29 gives Weeks pregnant 8 weeks, 3 days, Pregnancy week 9, Trimester First trimester, Your due date 2027-05-08.Source: ACOG Committee Opinion 700 (last period + 280 days)
  3. Based on Last period, First day of your last period 2026-01-05, Cycle length (days) 28, Work out on 2026-10-12 gives Weeks pregnant 40 weeks, Pregnancy week 41, Days until the due date 0, Trimester Third trimester.Source: ACOG Committee Opinion 700: day 280 is the due date

How it works

The calculator finds the start of the pregnancy count (day 0), from what you know:

  • Due date (the default): start = due date − 280 days
  • Last period: start = first day of the last period + (cycle length − 28) days, for a cycle of 21 to 45 days
  • Ultrasound: start = ultrasound date − (weeks × 7 + days) the scan gave
  • Conception date: start = conception date − 14 days

Then, on the work-out date (today unless you change it under "Use another date"):

  • Days pregnant d = work-out date − start.
  • Weeks pregnant = d ÷ 7 whole weeks and the remainder in days.
  • Pregnancy week = ⌊d ÷ 7⌋ + 1: week 1 is days 0 to 6, week 2 is days 7 to 13, and so on.
  • This week starts on start + 7 × (week − 1) days and ends 6 days later.
  • Trimester: "First trimester" for d under 98 (to 13 weeks 6 days), "Second trimester" under 196 (to 27 weeks 6 days), "Third trimester" under 294, then "Past 42 weeks (post-term)".
  • Due date = start + 280 days; days until the due date = 280 − d, shown up to and including the due date.

If the work-out date is before the start, the trimester row reads "Before the pregnancy count starts: check the dates"; if it is more than 300 days (42 weeks 6 days) after the start, it reads "More than 42 weeks 6 days after the pregnancy count starts: check the dates". In both cases the week, weeks pregnant, this week and days rows are left out, and the start and due date still show. All dates are calendar dates with no time of day.

The timeline shows the pregnancy from the start to the due date, with this week marked.

Assumptions

  • A pregnancy is counted from the first day of the last period and lasts 280 days to the due date (ACOG). Ovulation is 14 days before the next period, so a longer or shorter cycle moves the dates by the difference.
  • Real cycles and pregnancies vary; an early ultrasound dates a pregnancy best.
  • This calculator gives an estimate. It is not medical advice; your obstetric provider sets your dates.

Worked examples by hand

Due March 1, 2027, on September 29, 2026. Start = March 1, 2027 − 280 days = May 25, 2026. From May 25 to September 29, 2026 is 127 days = 18 weeks 1 day, so week 19. Week 19 started on day 126, September 28, 2026, and ends October 4, 2026. Second trimester; 280 − 127 = 153 days to go.

Last period August 1, 2026, 28-day cycle, on September 29, 2026. Start = August 1. 59 days = 8 weeks 3 days, week 9, first trimester. Due date = August 1 + 280 days = May 8, 2027.

Last period January 5, 2026, on October 12, 2026. 280 days = 40 weeks 0 days: the due date itself, the first day of week 41, third trimester, 0 days to go.

Other questions people ask

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.