Obstetrics - Tests of fetal well-being

 

Amniocentesis and chorionic villus sampling

Genetic disorders may be caused by a missing, damaged, or extra chromosome. Human beings have normally have a total of 23 pair of chromosomes making a total of 46. One chromosome from each pair is inherited from the mother, the other from the father. One pair represents the sex chromosomes, X and Y. Females have 2 X chromosomes, males an X and a Y. Consequently, mothers can contribute only an X chromosome to her infant, the fathers either an X or a Y. Major birth defects result when an error occurs during the formation of the egg or sperm or during the union of the sperm and egg, producing an infant with either an abnormal number of chromosomes or damaged chromosomes. Most children with chromosomal disorders have physical defects, and some have mental defects. Two examples of chromosomal disorders are:

• Down syndrome: a genetic disorder resulting from an extra copy of the 21st chromosome in which mental retardation, abnormal features of the face, and medical problems such as heart defects occur.
• Trisomy 18: a genetic disorder resulting from an extra copy of the 18th chromosome that causes serious problems with physical and mental development. Most infants with trisomy 18 die within the first year of life. The risk of having a child with a chromosomal disorder increases as a woman ages. For instance, a 35-year-old woman has a 1-in-192 (less than 1%) chance of having a baby with a chromosomal disorder. The chance increases to 1 in 66 (about 1.5%) in a woman aged 40 years.
  

 

An amniocentesis is a procedure in which a needle is used to withdraw a small amount of amniotic fluid and cells from the sac surrounding the fetus. The amniotic fluid and cells can be tested to detect certain chromosomal problems in the fetus. Amniocentesis usually is done at 15–20 weeks of pregnancy by one of our physicians.

 

Chorionic villus sampling: CVS is a procedure generally performed instead of an amniocentesis. During this procedure a small sample of cells is taken from the placenta with a needle usually placed through the vagina and tested. CVS detects some of the same chromosomal problems as amniocentesis does. This test can be performed earlier than amniocentesis however—often at 10–12 weeks of pregnancy. Patients requesting CVS can be referred to a medical center that performs this procedure on a regular basis.

 

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