Dating scan nuchal scan

The top line represents the skin and bottom one, represents the nasal bone.

A third line, almost in continuity with the skin, but at a higher level, represents the tip of the nose.

First trimester ultrasonic scans may show 'soft' markers for chromosomal abnormalities, such as an increased fetal nuchal translucency (back of the neck) to enable detection of Down's syndrome fetuses.

The test is performed between 11 weeks and 13 weeks 6 days into your pregnancy.

If the nasal bone is visible at the scan then this will reduce the chance of your baby having Down's syndrome.

The view of the nasal bone should show three distinct lines: The first two lines, are horizontal and parallel to each other, resembling an 'equal sign'.

At the first trimester scan they confirm that the fetus is alive, they assess the gestational age by measuring the crown-rump length and will look for any major problems.

This is a routine ultrasound examination done at 10 to 14 weeks of gestation.

It is called a 'translucency' because on ultrasound this appears as a black space beneath the fetal skin.This is normally less than 2.5mm and when seen increased (greater than 2.5mm, see image on the right) may indicate the baby has Down syndrome or may indicate another chromosomal abnormality. The nuchal translucency test will also check whether your baby has a visible nasal bone.In the past few years it has been seen that approximately 3 in 4 babies with Down's syndrome do not have a visible nasal bone at the time of the first trimester screening test.Occasionally the nasal bone cannot be seen at 11 weeks as it is too early in the pregnancy.If this is the case and this creates some concern then the scan can be repeated a week later.

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