How is Down syndrome diagnosed during a pregnancy?
A doctor will perform screening tests during the pregnancy in order to diagnose (and/or indicate a high risk of) Down syndrome of the fetus.
First trimester screening for Down syndome involves an ultrasound. In the ultrasound, they are looking for an increased space in the back of the fetal neck. An increased diameter is associated with chromosomal abnormalities. A blood test is performed to check for high levels of PAAP-A and hCG, two hormones that are produced by the fetus and passed into the mother's bloodstream. Women whose results are abnormal would be offered amniocentesis.
Also, researchers are now looking at whether the absence of a nasal bone in the fetus, shown by an ultrasound, may also indicate a higher risk of Down syndrome.
When Is It Done?
* Blood test between 9-11 weeks. * Ultrasound between 11-13 weeks. * Follow-up blood test and results reported between 16-18 weeks. Ultrasound and the blood test are simple (and the usual) procedures.
This involves a simple blood test (triple screen) which will measure three hormones produced by the fetus and is passed into the mother's bloodstream: alpha-fetoprotein (AFP), hCG, and estriol. Some doctors also measure the hormone inhibin-A (making it a quad screen). This blood test can also be used in conjunction with the first trimester screening, call "integrated screen", for the highest detection rate of chromosomal abnormalities but not all practitioners are equipped to test this way yet.
Abnormally low levels suggest an increased risk of Down syndrome, or other chromosomal defect. The triple screen cannot diagnose a birth defect; it can only indicate an increased risk. The false positive rate for the triple screen is extremely high. Any abnormal result means that further testing is required. This is done between the 15th and 18th weeks and results are available within one week.
Chorionic Villus Sampling (CVS) is performed in the first trimester. CVS is useful to detect disorders for which the technology exists such as Down syndrome. Occasionally both CVS and amniocentesis may be needed. It is performed between the 10th and 13th weeks of pregnancy. CVS is performed depending on the location of the placenta, the sample of cells is taken via the vagina and cervix or via a needle inserted in the abdominal wall. Test results are available in 3-5 days. Since the chorionic villi are of fetal origin, examining them can give a complete picture of the gentic makeup of the developing fetus.
Amniocentesis is more than 99 percent accurate in diagnosing, or ruling out which is much more likely, Down syndrome. It is recommended when:
* The mother is over 35 years old. * The couple has already had a child with a chromosomal abnormality. * The mother is a carrier of an X-linked genetic disorder. * Both parents are carriers of an autosomal recessive inherited disorder. * A parent is known to have a condition such as Huntington's chorea. * Fifth Desease or other fetal infection is suspected. * Results of a screen test turn out to be abnormal and evaluation of the amniotic fluid is necessary. * It is necessary to assess the maturity of the fetal lungs late in pregnancy. * The couple has another child or a close relative with a neural tube defect.