How Many Weeks Am I? Pregnancy Dating, and Why It Is the First Thing a Doctor Checks

Weeks are counted from your last period

Here is the part that catches everyone out. Pregnancy is dated from the first day of your last menstrual period, not from the day you conceived. Since ovulation happens roughly two weeks after a period starts, you are counted as two weeks pregnant before conception has even occurred.

So someone who says "I think I got pregnant three weeks ago" is usually around five weeks pregnant, not three. Nobody is doing the arithmetic wrong — the convention is simply counted from a different starting line.

Why dating comes before every decision

A first scan is not a formality on the way to something else. It answers three separate questions, and two of them are about your safety rather than your decision:

• Where the pregnancy is. An ectopic pregnancy — one growing outside the uterus — is a medical emergency and cannot be managed by any route that skips a scan.
• Whether it is viable. A proportion of early pregnancies have already stopped developing, which changes the conversation entirely.
• How many weeks. Which determines what is appropriate, and how straightforward it will be.

This is the reason a clinic will not plan anything from a home test and a remembered date. It is also the single strongest argument against ordering pills online: nobody in that transaction has answered the first question, and the first question is the one that can kill you.

What actually changes as weeks pass

People often imagine a single cut-off with nothing on either side of it. In practice the change is gradual, and it shows up in several ordinary ways at once:

• Which approaches are appropriate — the options narrow as weeks go on.
• How simple the procedure is — earlier generally means shorter and more straightforward.
• Recovery — earlier generally means less bleeding and a quicker return to normal.
• Preparation — later can mean additional steps beforehand and a longer appointment.
• Cost — later almost always costs more, for all of the reasons above.

None of this is a reason to panic about a few days. It is a reason not to spend three weeks reading forums before making an appointment, because those three weeks are the ones that actually change your options.

The legal position, stated honestly

Korea's Constitutional Court ruled the criminal provisions on abortion unconstitutional in 2019, and those provisions lost effect at the start of 2021. Replacement legislation setting out a clear framework has been debated since then without being settled, which leaves the country in an unusual position: the old criminal penalties no longer apply, and no new statutory scheme has taken their place.

What this means in practice is that the limits you will actually encounter are medical rather than criminal — set by what is safe and appropriate at a given stage, and by what an individual clinic provides. Those vary, which is why a general article cannot tell you your answer and a consultation can.

If you have no idea of your dates

Plenty of people genuinely cannot answer the question, and it is not a problem. Irregular cycles, hormonal contraception, breastfeeding, recent travel or simply not tracking all make the date unavailable — and an ultrasound does not need it.

• You don't remember your last period — say so; the scan dates the pregnancy independently.
• Your cycles are irregular or absent — common with PCOS, and a reason the scan is more reliable than the calendar anyway.
• You were on contraception — withdrawal bleeds are not periods, and dating from one can mislead.
• You've had bleeding since — spotting in early pregnancy is often mistaken for a light period, which is one of the most common ways people underestimate their weeks.

Bring whatever you have — an app, a note in your phone, a rough guess, or nothing at all. The useful thing to bring is not the date; it is yourself, sooner rather than later.