Pregnancy Week Calculator

About the Pregnancy Week Calculator

Pregnancy is tracked in weeks and days, and the question of how far along you are comes up constantly — at appointments, in apps, in conversations with family, and in every guide that describes what happens when.

The answer follows a precise convention, but there are two points of confusion built into it. One is whether "12 weeks 3 days" and "week 13" mean the same thing (they do). The other is that the count starts about two weeks before conception, so the pregnancy is always two weeks "older" than the baby.

This pregnancy week calculator gives the gestational age on any date, from either your last period or your due date, and shows the age since conception alongside it, so the numbers in a scan report and in a development guide can be matched up.

How to Use the Pregnancy Week Calculator

Choose whether you know the first day of your last period or your due date. If a scan has given you a due date, use that — it is usually more accurate than the period date, and it will match your clinic's count.

Leave on this date blank to see how far along you are today, or enter any date to see where you will be then, or where you were.

How the Weeks Are Counted

  gestational age  =  date − first day of last period
                   =  completed weeks × 7 + days

If you enter a due date instead, the calculator first works out the equivalent start date by counting back 280 days.

The result is expressed in completed weeks and days. That is the clinical convention, and it is what appears on scan reports and maternity notes.

Step-by-Step Example

Last period started 1 March 2026. Checking on 24 September 2026.

  1 March → 24 September  =  207 days
  207 ÷ 7                 =  29 weeks, remainder 4

So 29 weeks 4 days — in week 30, and in the third trimester. The due date is 6 December, 73 days away, and the pregnancy is about 74% of the way there. The baby has been developing for about 27 weeks 4 days since conception.

Completed Weeks and "Week Number"

This is the source of most apparent disagreements between apps.

29 weeks 4 days means 29 full weeks have passed and you are four days into the thirtieth. So the same day can be described as "29 weeks" (completed) or "week 30" (current). Neither is wrong. Clinical notes use completed weeks and days; many apps and books use the current week number.

The calculator gives both: the weeks-and-days figure as the headline, and the week number beside it.

Pregnancy Age and Baby's Age

Gestational age starts on the first day of the last period, which is about two weeks before ovulation. So:

Scan reports, screening tests and appointment schedules all use gestational age. Some embryology and development resources use the age since conception, which is why a guide might describe something at "week 6" that your scan describes at eight weeks.

Why Months Do Not Work Well

People naturally think of pregnancy as nine months, but months are an awkward unit for it. They vary from 28 to 31 days, and forty weeks is 280 days, which is closer to nine and a quarter months than nine.

The calculator gives an approximate month figure using the average month of 30.44 days, but weeks and days are the only precise measure, which is why they are used throughout maternity care.

Trimesters

The first trimester runs to the end of week 13, the second from 14 weeks to the end of week 27, and the third from 28 weeks until birth. A few sources draw these boundaries a week differently, but these are the ones most guidelines and scan schedules follow.

From 37 weeks a baby is considered term, and from 42 weeks post-term.

Understanding Your Result

Weeks pregnant is the gestational age in completed weeks and days.

Week and trimester gives the current week number and trimester, and the term category from 37 weeks.

Due date shows how many days are left, or how many have passed, with the percentage of the way through.

Since conception shows the approximate age of the embryo or baby.

In months gives a rough month figure.

Worth knowing covers early scans, term, and what to do past the due date.

When Should You Use This Calculator?

To check how far along you are today, or on the date of an appointment.

To reconcile an app with your scan report when they seem to disagree.

To plan — for example, when you will reach 20 weeks or 37 weeks.

Not in place of your clinic's dating, which may reflect a scan.

Common Mistakes

Mixing up completed weeks and week number. 12 weeks 3 days is week 13.

Counting from conception. The count starts about two weeks earlier.

Using the period date after a scan has changed it. Use the scan's due date.

Thinking in months. Weeks and days are the precise unit.

Expecting an early scan to show everything. A heartbeat is usually visible from about six to seven weeks.

Questions about your own pregnancy are best taken to your midwife or doctor. Nothing here is medical advice.