Pregnancy Weeks and Trimesters Explained

Pregnancy is measured in weeks and days from a starting point that predates conception, which is why "six weeks pregnant" rarely means what people first assume. This guide explains the counting convention and where the trimester boundaries sit.

Gestational age counts from your last period

Gestational age is measured from the first day of your last menstrual period — the same starting point used to estimate your due date:

gestational age = today's date − first day of last menstrual period

It is expressed as weeks and days, written like 20w 5d. A full-term pregnancy is counted as 40 weeks from that date, which is where the 280 days in how due dates are calculated comes from.

The consequence is that the first two weeks or so of "pregnancy" occur before conception. At the moment of conception you are already counted as roughly two weeks pregnant. This is a convention chosen for practicality — the last period is a date most people can identify, while conception usually is not.

Our Pregnancy Week-by-Week Calculator shows your current gestational age, and the Pregnancy Due Date Calculator shows both the date and how far along you are.

The trimester boundaries

Trimesters are defined by ACOG as:

They are not exactly equal thirds, and the boundaries are conventions for organising care and discussion rather than sharp biological transitions. Nothing changes overnight when you cross from 13w 6d to 14w 0d.

A worked example

If your last menstrual period began on 1 April 2026 and today is 24 August 2026, that is 145 days:

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

That places you in the second trimester, with an estimated due date of 6 January 2027.

Why your week count might change

If an early scan revises your due date, your gestational age shifts with it — the two are calculated from the same starting point, so adjusting one adjusts the other.

A revision like this means your dating has been improved, not that something has gone wrong. As covered in how accurate is your due date, a first-trimester ultrasound dates a pregnancy more accurately than the last-menstrual-period method, so a scan-based date supersedes a calculated one.

What we deliberately do not tell you

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 We do not publish those here, because doing it responsibly means citing clinical sources for every claim, and because the range of normal is wide enough that generic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

For what to expect at your stage, your midwife or doctor, or your national health service's pregnancy guidance, will serve you better than a calculator's summary. Anything that concerns you is a question for them.

The bottom line

Gestational age is counted in weeks and days from the first day of your last period, which is why the count starts about two weeks before conception. ACOG places the first trimester at 0w0d–13w6d, the second at 14w0d–27w6d, and the third from 28w0d. If a scan has revised your dates, use the revised ones.