Trisomy 18 - Testing

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How is trisomy 18 diagnosed?

In newborns, chromosome analysis can be carried out to determine the exact cause of birth defects. Prenatal testing for trisomy 18 is possible.

Screening tests such as measurement of alpha fetoprotein (AFP) levels (sometimes referred to as a triple screen) is one screening test. The triple test screens the mother's blood for AFP, hCG (human chorionic gonadotropin, the so-called pregnancy hormone), and estriol (a type of estrogen). The test is performed between the 15th and 17th week of pregnancy. A positive result on a screening test does not mean that the baby will have trisomy 18 or any chromosomal abnormality. In fact, only about 11% of those women who receive a positive result for trisomy 18 in this test will actually have an affected fetus.

Ultrasound is another commonly used screening test. Like the AFP test, a simple ultrasound cannot be used to establish the diagnosis of trisomy 18. A more detailed ultrasound may be performed to look for the characteristic signs of trisomy 18, but it cannot confirm alone that trisomy 18 is present.

Analysis of fetal chromosomal material obtained during amniocentesis or chorionic villus sampling is necessary to prove that the additional copy of chromosome 18 is present. An amniocentesis is usually performed at 15-18 weeks of pregnancy and is the most commonly used test for the prenatal diagnosis of trisomy 18. During this procedure, a thin needle is inserted through the abdominal wall and a small sample of amniotic fluid is taken.

Chorionic villus sampling is another type of test that allows the examination of fetal genetic material. It is performed earlier in pregnancy (at 10-12 weeks after the last menstrual period) and therefore carries the advantage of allowing for an earlier diagnosis. This procedure involves the collection a chorionic villus cell sample from the placenta either through the insertion of a needle in the abdominal wall or through a catheter in the vagina.

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See what others are saying

Comment from: erika rodriguez, Female (Caregiver) Published: May 09

Last month I lost my baby. He has trisomy 18. The doctor did an ultrasound and discovered it. I still do not understand why this happened to me and my husband.

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Comment from: makeuadress, 19-24 Male (Caregiver) Published: July 17

I have a son who is 23 years old. In 1990, he was diagnosed with partial trisomy 18.He has a translocation with his first chromosome. He has a VSD, seizures, and failure to thrive syndrome. He requires total care and is unable to walk, talk, or sit without assistance. But he is a very happy child. I allowed him to attend public school, and he graduated from high school in 2012. There is not a lot he can do, but we have always taken care of him. His food has to be finely chopped and sometimes pureed. He weighs 25 pounds and is a little over 3 feet tall. But he is the joy of our life. I didn't know he had the problem until after he was born. Doctors told me he might live three months. But we have done everything in our power to keep him well and to make his life fulfilling.

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