Your health care provider uses abdominal ultrasound (not vaginal) to measure the nuchal fold. All unborn babies have some fluid at the back of their neck. In a baby with Down syndrome or other genetic disorders, there is more fluid than normal. This makes the space look thicker.

A blood test of the mother is also done. Together, these 2 tests will tell if the baby could have Down syndrome or another genetic disorder.

Having a full bladder will give the best ultrasound picture. You may be asked to drink 2 to 3 glasses of liquid an hour before the test. DO NOT urinate before your ultrasound.

You may have some discomfort from pressure on your bladder during the ultrasound. The gel used during the test may feel slightly cold and wet. You will not feel the ultrasound waves.

Your provider may advise this test to screen your baby for Down syndrome. Many pregnant women decide to have this test.

Nuchal translucency is usually done between the 11th and 14th week of pregnancy. It can be done earlier in pregnancy than amniocentesis. This is another test that checks for birth defects.

A normal amount of fluid in the back of the neck during ultrasound means it is very unlikely your baby has Down syndrome or another genetic disorder.

Nuchal translucency measurement increases with gestational age. This is the period between conception and birth. The higher the measurement compared to babies the same gestational age, the higher the risk is for certain genetic disorders.

The measurements below are considered low risk for genetic disorders:

• At 11 weeks -- up to 2 mm
• At 13 weeks, 6 days -- up to 2.8 mm

More fluid than normal in the back of the neck means there is a higher risk for Down syndrome, trisomy 18, trisomy 13, Turner syndrome, or congenital heart disease. But it does not tell for certain that the baby has Down syndrome or another genetic disorder.

If the result is abnormal, other tests can be done. Most of the time, the other test done is amniocentesis.

There are no known risks from ultrasound.