Which invasive procedures are performed in prenatal screening of Down syndrome?

Updated: Apr 30, 2018
  • Author: Gratias Tom Mundakel, MBBS, DCH; Chief Editor: Maria Descartes, MD  more...
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Answer

Amniocentesis, routinely performed at 14-16 weeks’ gestation, remains the criterion standard of invasive diagnostic tests. Testing for chromosomal disorders is 99.5% accurate. Rare cases of mosaicism are missed, and results can be inaccurate if maternal-cell contamination occurs. The procedure is associated with a small risk of pregnancy loss (1:200-300).

Chorionic villus sampling (CVS) is performed at 10-13 weeks’ gestation; earlier testing is thought to be associated with a 1 in 300-1000 risk of fetal transverse limb deficiency, a small risk of maternal cell contamination, and a 0.5-1% risk of a fetal loss after the procedure. The accuracy of CVS (96-98%) is less than that of midtrimester amniocentesis, because of confined placental mosaicism and maternal-cell contamination.

Percutaneous umbilical blood sampling (PUBS) is approximately 95% successful in obtaining a blood sample for cytogenetic testing. The pregnancy-loss rate is 3.25% for PUBS done for chromosomal indications, compared with 1.25% and 2.75% for PUBS done for nonchromosomal indications. The indication for the procedure greatly increases the risk of procedure-related pregnancy loss.

The availability of in vitro fertilization has allowed preimplantation diagnosis of single-gene disorders, sex selection for X-linked disorders, and identification of chromosomal aneuploidies. After a biopsy sample is obtained from the first polar body, the blastocyst, or the 6-cell to 8-cell embryo, FISH can then be used to diagnose fetal aneuploidy. However, standard cytogenetic confirmation is not possible for the preimplantation diagnosis.


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