How Many Weeks Pregnant Am I?

How pregnancy weeks are counted

How far along a pregnancy is — its gestational age — is measured from the first day of the last normal menstrual period, and expressed in completed weeks and days.¹ The first day of that period counts as day zero, not day one.¹

So on the first day of your last period you were 0 weeks 0 days, a week later 1 week 0 days, and 87 days after it 12 weeks 3 days. This site always shows gestational age this way: completed weeks, then days.

What “12 weeks 3 days” means

Gestational age counts completed weeks.¹ “12 weeks 3 days” means 12 full weeks have passed since the first day of your last period, plus 3 more days. It is the next day after 12 weeks 2 days, and 4 days later it becomes 13 weeks 0 days.

Working it out from your last period

1. Write down the first day of your last period. Example: 9 July 2026.
2. Count the days from that date to today, with the first day as day zero. Example: from 9 July to 4 October 2026 is 87 days.
3. Divide by 7. 87 ÷ 7 is 12, with 3 left over — so 12 weeks 3 days.

Counting days across several months by hand is fiddly, so the Pregnancy Weeks Calculator does it for you, along with the days remaining until your estimated due date.

Here is the same example pregnancy at a few points along the way:

Working it out from your due date

If you already have a due date — for example from a scan — you can count from that instead. By convention, the due date is 280 days, or 40 weeks 0 days, after the first day of the last period.² So:

• if your due date is 10 weeks away, you are 30 weeks 0 days pregnant
• if your due date is 15 April 2027, then on 4 October 2026 it is 193 days away and you are 12 weeks 3 days pregnant

The rule is the same every time: subtract the time remaining from 40 weeks 0 days. If your due date came from a scan, counting from it gives a gestational age that matches your scan dating. Don’t have a due date yet? You can calculate your estimated due date first.

Why you’re counted as pregnant before conception

Pregnancy is dated from the first day of the last normal menstrual period.¹ That first day is a date you can note down; the day of conception usually isn’t known, and the dating convention simply places it 14 days later.² Starting from the period gives everyone the same fixed point to count from.

Because counting starts at the last period, and the dating convention assumes ovulation on day 14 of a regular 28-day cycle,² the first two weeks counted are, by convention, before conception.

That’s why the numbers can feel surprising. In the example above, a conception date estimated by convention is 23 July 2026 — and on that day the pregnancy is already dated as 2 weeks 0 days. A month later, on 23 August 2026, it is 6 weeks 3 days. Read more about how a conception date is estimated.

What “40 weeks” means

40 weeks 0 days is the estimated due date: by convention, 280 days after the first day of the last period.² It’s also the average length of a singleton pregnancy measured from that day.³

But 40 weeks is a reference point, not an expected birthday. Pregnancy normally lasts from 37 to 42 weeks,⁴ and in one study only 5% of births happened on the estimated due date.⁵ Read more about how accurate a due date is.

Milestones in weeks

Weeks and days are also how term is defined. ACOG uses these categories:³

The WHO counts days 280 to 286 after the first day of the last period as 40 completed weeks.¹ A baby born before 37 completed weeks of pregnancy is described as preterm.⁶

When the answer changes

Ultrasound measurement up to and including 13 weeks 6 days is the most accurate method to establish or confirm how far along a pregnancy is.² If an early scan changes your due date, how many weeks pregnant you are changes with it. Read about ultrasound vs LMP dating.

If you don’t know or aren’t sure of the first day of your last period, speak to a midwife or GP.⁴

Frequently asked questions

How far along am I based on my due date?

Your due date is 40 weeks 0 days.² Subtract the number of weeks and days until it from 40 weeks: 10 weeks before the due date is 30 weeks 0 days.

How am I 4 weeks pregnant if I conceived 2 weeks ago?

Pregnancy is counted from the last period, and by convention conception is placed 14 days after it.² So 2 weeks after conception is counted as 4 weeks 0 days.

Is 40 weeks the same as full term?

40 weeks 0 days is your estimated due date. ACOG’s full-term category runs from 39 weeks 0 days to 40 weeks 6 days.³

Does my week count change if my due date changes?

Yes. Weeks and days are counted from the same starting point as the due date, so if an early scan moves your due date, how far along you are moves with it.²

Why might another tool show a slightly different answer?

Tools can differ in how they count. This site counts completed weeks and days from the first day of the last period as day zero, as in WHO definitions.¹ If your healthcare team has given you dates, use theirs.

Medical statements in this article are linked to the numbered sources listed below: published clinical guidance and peer-reviewed research. It has not been reviewed by a named clinician. Read how content is written in our editorial policy, and see our medical disclaimer.