It is this black space that you will see measured during the ultrasound scan.
Currently the most accurate non invasive test for detecting Down syndrome during pregnancy is the measurement of the nuchal translucency with an ultrasound between 11 to 14 weeks of pregnancy.
The 12 week scan is a routine ultrasound examination carried out at 10 to 14 weeks of gestation.
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 also look for any major problems.
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.
A scan maybe ordered when an abnormality is suspected on clinical grounds.
Otherwise a scan is generally booked in the first trimester to confirm pregnancy, exclude ectopic or molar pregnancies, confirm cardiac pulsation and measure the crown to rump length for dating.
A third line, almost in continuity with the skin, but at a higher level, represents the tip of the nose.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'.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.