Tracking Your Pregnancy Week by Week

"How many weeks are you?" is the standard question of pregnancy, and the answer follows a specific counting convention. This guide covers how the number is worked out and what it is for.

The calculation

Gestational age is the time elapsed since the first day of your last menstrual period:

gestational age = today − first day of last menstrual period

It is reported as weeks and days — written like 20w 5d — because a week alone is too coarse for a 40-week process where appointments and screening windows are scheduled to within a few days.

For a last period beginning 1 April 2026, checked on 24 August 2026, that is 145 days:

145 ÷ 7 = 20 weeks, remainder 5 days  →  20w 5d

Our Pregnancy Week-by-Week Calculator works this out for any date, along with your estimated due date and trimester.

Counting starts before conception

The convention counts from your last period, which is roughly two weeks before conception occurs. So at conception you are already "two weeks pregnant" by this measure.

That seems odd until you consider the alternative: the date of conception is rarely known, while the start of a period usually is. The convention trades biological precision for something people can actually report. Pregnancy weeks and trimesters explained covers this and the ACOG trimester boundaries in more detail.

What the number is useful for

• Scheduling. Antenatal appointments and screening tests have windows defined in gestational weeks.
• Context for your care. Clinical decisions are made relative to gestational age, so it is the shared reference point between you and your maternity team.
• Orientation. Knowing roughly where you are in the process, and roughly how long remains.

It shares its starting point — and therefore its uncertainty — with your estimated due date. If your due date is revised, your week count moves with it.

If a scan has revised your dates

A first-trimester ultrasound dates a pregnancy more accurately than the last-menstrual-period method, so if a scan has given you different dates, those supersede a calculated figure — and your week count should follow the revised date.

This is your dating being improved, not a sign that anything has gone wrong. How accurate is your due date covers the reasoning.

What we don't publish

You will find week-by-week accounts everywhere of what is developing and what you might be feeling. We do not publish those, for two reasons: doing it responsibly requires citing clinical sources for every claim, and the range of normal is wide enough that generic week-by-week descriptions can worry people without informing them.

Your midwife or doctor, or your national health service's pregnancy guidance, is a better source for what to expect at your stage — and the right place to take anything that concerns you.

The bottom line

Gestational age counts from the first day of your last period, reported as weeks and days, which is why the count begins about two weeks before conception. Use the Pregnancy Week-by-Week Calculator to find yours, defer to any scan-revised dates, and take questions about what to expect to your care team.