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For many women, ultrasound scans are the highlight of pregnancy.

At around seven weeks of pregnancy, it’s reasonably common for women to have their first ultrasound.

This can be for many reasons but the most common is to confirm pregnancy and to check that the embryo is viable and “everything’s ok”.

Having a scan in pregnancy is usually a happy event, but be aware that ultrasound scans may detect some serious abnormalities, so you should be prepared for that information.

See What if a screening test shows a possible problem?

You can ask your midwife or doctor before the scan if this is the case. Find out more about what happens during a pregnancy ultrasound scan.

The purpose of the dating scan is to: Some abnormalities may also be detected at this scan, such as neural tube defects (spina bifida is a type of neural tube defect).

The limbs and the yolk sac, though obviously important, are not the primary means of measuring growth.

An average length of the embryo at 7 weeks is anywhere between 5mm-12mm. Obviously, every pregnancy is unique and individual factors influence the size of the embryo at this early stage, and the embryo shows development week by week.

Not all units offer an early scan if all is going well in this pregnancy, though.

Ask your GP if you want the reassurance of an early scan. The scan's main purpose is to work out accurately how many weeks pregnant you are and estimate your due date.

Other reasons to have a 7 week ultrasound are to: The embryo will be measured from the top of its head, the “crown” to its bottom or “rump”.