. The dating scan.
This is done between eight and twelve weeks of pregnancy. By measuring the fetus from the top of its head to the end of its bottom (the crown rump measurement), the Sonographer can calculate how many weeks old the fetus is, and therefore can give an estimated date of delivery (EDD). This scan is usually performed in combination with
. The nuchal translucency test.
This is one of the most accurate tests used in screening for Downs Syndrome. It measures the amount of fluid in the nuchal fold (the two layers of skin behind the fetus' neck). A fetus with a greater collection of fluid is more likely to have Downs Syndrome. The result of this test gives a risk factor and not a definitive positive or negative result. For example you may be given a result that says your baby has a 1:500 chance of having Downs Syndrome. A woman who is given a 1:250 or less screening risk would normally be counselled about further diagnostic tests available.
. The anomaly scan.
This is performed between 18 and 22 weeks of pregnancy. A series of measurements are taken to ensure that the baby is the correct size for the stage of pregnancy. The baby is checked for any physical abnormalities, and the sex can often be determined if the baby is in an accommodating position. The size, position and function of the placenta (afterbirth) will also be checked.