Trisomy 18 (cont.)

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What are the characteristic signs and symptoms of trisomy 18?

Common birth defects in infants with trisomy 18 include a low birth weight; a small jaw and mouth; an abnormally shaped, small head; overlapping fingers and clenched fists; “rocker-bottom” feet; low-set ears; and defects in the heart and other organs. Surviving infants may display poor feeding, respiratory problems, delayed growth, and other life-threatening complications. Mental impairment is also characteristic of this syndrome.

How common is trisomy 18?

Trisomy 18 occurs on average in 1 out of every 6,000 babies. Girls are affected far more commonly than boys; about 80% of affected infants are female. Any woman can have a child with trisomy 18, but the risk increases with increasing maternal age.

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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