Tests performed to diagnose chromosomal abnormalities during pregnancy can be assessed in two sub headings. First group consists of procedures allowing definite diagnosis such as amniocentesis, chorion villus sampling and cordocentesis. Second group consists of ultrasonographic examination and biochemical tests assessing risk. Every woman at every age can give birth to a child with a chromosomal abnormality. Most common abnormality is trisomy 21 known as Down's syndrome. The syndrome is associated with mental retardation and other body defects.
Woman with a normal screening test and ultrasonographic examination have a reduced risk of giving birth to a child with Down's syndrome.
First trimester screen : The test is composed of two parts. First part consists of measurement of nuchal translucency. Second part is measurement of serum levels of PAPP-A and free beta hCG. It should not be forgotten that screening tests only assess if there is an increased risk and do not give a definite diagnosis. If the calculated risk is above the risk of amniocentesis, a diagnostic procedure is advised.
Second trimester screen : It is based on measurement of serum levels of AFP, hCG and estriol. AFP is also used for assessing risk of neural tube defect. If the calculated risk is above the risk of amniocentesis, a diagnostic procedure is advised.
Amniocentesis : The procedure is sampling the amniotic fluid. It is performed between 16 and 18 weeks. It is a simple procedure. First, localizations of the placenta and fetus are determined via ultrasonography. Then, a thin needle is inserted under USG guidance and 20 cc amnitic fluid is withdrawn. Sometimes, placenta must be passed by the needle. This has not been shown to increase fetal loss but some amount of hemorrhage is inevitable. A small amount of amniotic fluid leak can be seen.
What to do if the amniocentesis reveals chromosomal abnormality? Medical abortion is proposed to such families. If the family decides to go on the pregnancy, nothing is done. But usually abortion is preferred.
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