The dating scan is usually the first time you see your baby, and for most women it is the moment pregnancy starts to feel real. It is also the single most useful piece of information in your antenatal care.

When it is done

The dating scan is performed between 8 and 12 weeks of pregnancy. This window gives the most accurate measurement while still allowing plenty of time to plan the rest of your care.

A scan earlier than this — from around 6 weeks — is sometimes done for a specific reason such as bleeding, pain or a previous ectopic pregnancy. That is a viability scan rather than a dating scan.

What it checks

  • That the pregnancy is in the womb — ruling out an ectopic pregnancy, which is the most important thing an early scan does
  • Your due date — measured from the crown–rump length, the distance from the top of the head to the bottom of the spine
  • The heartbeat
  • How many babies there are — and if more than one, whether they share a placenta, which affects how the pregnancy is monitored
  • The ovaries and womb, including any fibroids or cysts that may be relevant later

How accurate is it?

Very. In the first trimester, dating by ultrasound is accurate to within about 3 to 5 days. Babies grow at a remarkably similar rate this early, which is exactly why this window is used.

Accuracy falls as pregnancy progresses, because babies grow at increasingly different rates. A scan at 30 weeks is a poor way to establish dates — which is why the early one matters.

If the scan date differs from your own

This happens often and is not a cause for concern. It usually means you ovulated earlier or later than the standard calculation assumes, which is entirely normal if your cycle is not exactly 28 days.

Where the two differ significantly, the scan date is normally used from that point onward, and your appointments and screening will be based on it.

What to expect on the day

The scan is usually done through the abdomen. You will be asked to arrive with a full bladder, which pushes the womb into a better position for imaging. It is genuinely uncomfortable — drink the amount you are told, not more, and it will be over in about 15 to 20 minutes.

Gel is applied to your abdomen and a handheld probe is moved across it. It is painless.

Sometimes a transvaginal scan gives a clearer picture — particularly early in the window, or if the womb is tilted. This needs an empty bladder. A slim probe is gently inserted into the vagina. It is safe in pregnancy and no more uncomfortable than an internal examination, though nobody enjoys being asked unexpectedly.

One thing worth knowing: the sonographer may go quiet while measuring. This is concentration, not bad news. They will explain what they can see once they have finished.

Is ultrasound safe?

Yes. Ultrasound uses sound waves rather than radiation, and has been used in pregnancy for decades with no evidence of harm to mother or baby when used appropriately by trained staff.

An honest note

Most scans are straightforward and happy. Occasionally one brings unexpected news — that the pregnancy is not developing as hoped, or is further along or less far along than expected.

You are welcome to bring your partner or someone else with you, and many women prefer to. If something unexpected is found, you will be given time, an explanation, and a clear plan for what happens next.

The bottom line

The dating scan at 8 to 12 weeks confirms the pregnancy is in the right place, checks the heartbeat and the number of babies, and gives the most accurate due date you will get. Arrive with a full bladder, expect about 20 minutes, and do not read anything into a quiet sonographer.

References

  1. National Health Service. Ultrasound scans in pregnancy [Internet]. NHS; [cited 2026 Jul 22]. Available from: https://www.nhs.uk/pregnancy/your-pregnancy-care/ultrasound-scans/
  2. National Health Service. Your antenatal appointments [Internet]. NHS; [cited 2026 Jul 22]. Available from: https://www.nhs.uk/pregnancy/your-pregnancy-care/your-antenatal-appointments/
  3. World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience. Geneva: WHO; 2016. Available from: https://www.who.int/publications/i/item/9789241549912