How many weeks pregnant am I? How gestational age is determined

The short answer

Clinicians count pregnancy from the first day of your last menstrual period (often shortened to LMP), not from the day you conceived. The number of weeks counted this way is called the gestational age.

South African law uses the same starting point. The Choice on Termination of Pregnancy Act defines the gestation period as the period of pregnancy calculated from the first day of the last menstrual period.

Why it is counted from your last period

Most people know roughly when their last period started, but few know exactly when conception happened. Counting from the last period gives a consistent starting point.

It also produces a result that surprises people: because ovulation typically happens around two weeks after a period begins, you are usually considered about two weeks “more pregnant” than the time since conception. Someone who conceived three weeks ago would generally be described as about five weeks pregnant.

How a clinician works it out

• Your dates. The clinician asks when your last period began and how regular your cycle is.
• Your history and symptoms. These help to check whether the dates make sense.
• Examination or ultrasound. If your dates are uncertain, your periods are irregular, or the clinician needs more information, they may recommend a scan or an examination.

A home pregnancy test confirms that you are probably pregnant. It cannot tell you how many weeks.

What if I do not know my dates?

That is common, and it is not a barrier to care. Tell the clinician what you do know: an approximate date, when you last had unprotected sex, or when you first had a positive test. They will decide how best to establish the gestational age.

Why the number of weeks matters

Gestational age shapes almost every part of abortion care:

• Method. Which methods are clinically appropriate depends on the stage of pregnancy. See medical vs surgical abortion.
• The law. Different legal conditions apply in the first 12 weeks, from week 13 to week 20, and after week 20. See termination of pregnancy law in South Africa.
• Who may provide care. The law distinguishes between stages here too.

What this means for you

If you think you might be pregnant and are unsure what you want to do, finding out how far along you are is a sensible first step. It does not commit you to anything. A pregnancy consultation gives you that information and an explanation of your options.