Why It Is Done

Karyotyping is done to:

• Find out whether the chromosomes of an adult have a change that can be passed on to a child.
• Find out whether an extra or missing chromosome is preventing a woman from becoming pregnant or is causing miscarriages.
• Find out whether an extra or missing chromosome is present in a fetus. Karyotyping also may be done to find out whether chromosomal problems may have caused a fetus to be stillborn.
• Find out the cause of a baby’s birth defects or disability.
• Help determine the appropriate treatment for some types of cancer.
• Identify the sex of a person by checking for the presence of the Y chromosome. This may be done when a newborn’s sex is not clear.

How To Prepare

You do not need to do anything before you have this test.

Talk to your doctor about any concerns you have about the need for the test, its risks, or how it will be done. To help you understand the importance of this test, fill out the medical test information form( What is a PDF document? ).

Since the information obtained from karyotyping can have a profound impact on your life, you may want to see a doctor who specializes in genetics (geneticist) or a genetic counselor. This type of counselor is trained to help you understand what karyotype test results mean for you, such as your risk for having a child with an inherited (genetic) condition like Down syndrome. A genetic counselor can help you make well-informed decisions. Ask to have genetic counseling before making a decision about a karyotype test.

How It Is Done

Karyotype testing can be done using almost any cell or tissue from the body. A karyotype test usually is done on a blood sample taken from a vein. For testing during pregnancy, it may also be done on a sample of amniotic fluid or the placenta.

Blood sample from a vein

The health professional drawing your blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Apply a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure on the site and then put on a bandage.

Cell sample from a fetus

For this type of test, cells are collected from the fetus using amniocentesis or chorionic villus sampling.

Cell sample from bone marrow

Bone marrow aspiration may be used for a karyotype test. To learn more about how this test is done, see the topic Bone Marrow Aspiration and Biopsy.

How It Feels

Blood sample from a vein

You may feel nothing at all from the needle puncture, or you may feel a brief sting or pinch as the needle goes through the skin. Some people feel a stinging pain while the needle is in the vein. But many people do not feel any pain, or have only minor discomfort, once the needle is positioned in the vein. The amount of pain you feel depends on the skill of the health professional drawing the blood, the condition of your veins, and your sensitivity to pain.

What Affects the Test

If you are being treated for cancer, the results of a karyotype test may not be accurate. Chromosomes may be damaged by some types of cancer treatment.

What To Think About

• If the results of karyotype are abnormal, other family members may be advised to have the test.
• A sample taken by gently swabbing the tissues inside the cheek (called a buccal swab) sometimes is used for a karyotype test. But results from buccal swabbing are less accurate than other types of karyotype tests.
• Sometimes a karyotype test is combined with other genetic tests to provide more specific information about genetic problems. To learn more, see the topic Genetic Test.