Pregnancy Week Calculator

Pregnancy is counted in weeks and days, written as something like 12w3d. This calculator converts your last period or due date into that figure for today's date.

Enter whichever you have. The answer updates to the current day each time you use it, so it stays right through the pregnancy.

These calculators provide estimates for informational purposes only. They are not intended to diagnose, treat, prevent, or replace professional medical advice. Pregnancy and menstrual cycles can vary, so results may not be exact.

How this calculator works

What you need to enter:

The first day of your last menstrual period, or your estimated due date

Counting from your last period

Gestational age is the number of days between the first day of your last menstrual period and today. Divide that by seven and you have completed weeks; the remainder is the extra days.

So 85 days is 12 weeks and 1 day, written 12w1d. The week number only advances once a full seven days have passed, which is why you spend seven days at each week.

Why week 12 means eleven completed weeks

This is the part that confuses almost everyone. Saying you are 12 weeks pregnant means you have completed 12 weeks and are living through the thirteenth. It works the same way as age: a 12-year-old has finished twelve years and is in their thirteenth.

Phrases like "in your twelfth week" therefore mean something different from "12 weeks pregnant". This calculator always reports completed weeks and days, the same convention used in maternity care.

Working back from a due date

If you know your due date but not your last period, the calculator subtracts 280 days from the due date to recover the equivalent starting point, then counts forward from there.

This matters when a scan has already revised your due date. Entering the revised due date gives a gestational age that matches what your care team is using, whereas entering an old last period date would not.

A worked example

Last period on 26 June 2026, and today is 19 September 2026.

Count the days from 26 June to 19 September: 85 days.
85 divided by 7 gives 12 whole weeks, remainder 1.
That is 12 weeks and 1 day, or 12w1d.
Week 12 is below week 14, so this is the first trimester, and the due date on 2 April 2027 is 195 days away.

Tomorrow the same pregnancy is 12w2d. The week number will roll to 13 on 26 September 2026.

What this calculator can't tell you

The figure is only as good as the date you enter. An uncertain last period gives an uncertain week count.
If a scan revised your due date, use that revised due date rather than your last period, or the two figures will disagree.
The calculator reports where you are in a standard 40-week count. It says nothing about how a particular pregnancy is developing.

What to do next

If you do not have a due date yet, work one out with the due date calculator.
See the milestone dates for the weeks ahead in the pregnancy calculator.

Common questions

How many weeks pregnant am I?

Enter the first day of your last period, or your due date, and the calculator gives the answer in completed weeks and days for today. Counting by hand: take the days since your last period, divide by seven, and the whole number is your week with the remainder as days.

Am I 12 weeks pregnant or in my twelfth week?

Those describe different points. Being 12 weeks pregnant means twelve full weeks are behind you and you are in your thirteenth. Being in your twelfth week means you are somewhere between 11w0d and 11w6d. Clinical notes always use the first form, which is what this calculator reports.

How many months pregnant is that?

The calculator shows the month alongside your week count. It is approximate by nature: weeks do not divide evenly into months, which is why care providers work in weeks. The common nine-month table puts weeks 9 to 13 in month 3 and weeks 28 to 31 in month 7, and you will find sources that draw those lines a week either way.

How many weeks are left until my due date?

The calculator shows the days remaining alongside your current week. A 40-week count means the days left are 280 minus your gestational age in days, which it works out for you.

My scan gave a different number of weeks. Which is right?

A first-trimester scan measures the pregnancy directly and is generally more accurate than counting from a remembered date. If they disagree, your care team's dating is the one to use. Enter the due date they gave you and this calculator will match it.