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Amniocentesis or chorionic villus sampling is necessary to conclusively diagnose the majority of genetic disorders, with amniocentesis being the gold-standard procedure after 15 weeks' gestation. [3] Transabdominal chorionic villus sampling is an alternative to amniocentesis if genetic diagnostic testing is to be performed in the first ...
An amniotic fluid sample is collected via amniocentesis and the sample is spun down in a centrifuge at 1000 rpm for 3–5 minutes. Thin layer chromatography (TLC) is performed on the supernatant, which separates out the components. Lecithin and sphingomyelin are relatively easy to identify on TLC and the predictive value of the test is good. [4]
It entails sampling of the chorionic villus (placental tissue) and testing it for chromosomal abnormalities, usually with FISH or PCR. CVS usually takes place at 10–12 weeks' gestation, earlier than amniocentesis or percutaneous umbilical cord blood sampling. It is the preferred technique before 15 weeks. [2]
Amniotic fluid is removed from the mother by an amniocentesis procedure, where a long needle is inserted through the abdomen into the amniotic sac, using ultrasound guidance such that the fetus is not harmed. Amniocentesis is a low risk procedure, with risk of pregnancy loss between 1 in 1,500 – 1 in 700 procedures.
This technique in conjunction with FISH, m-FISH can produce more reliable results, since analysis is done on whole metaphase plates [49] In addition to FISH and PCR, single cell genome sequencing is being tested as a method of preimplantation genetic diagnosis. [50] This characterizes the complete DNA sequence of the genome of the embryo.
The results of the blood test are then combined with the NT ultrasound measurements, maternal age, and gestational age of the fetus to yield a risk score for Down syndrome, trisomy 18, and trisomy 13. First Trimester Combined Test has a sensitivity (i.e. detection rate for abnormalities) of 82–87% and a false-positive rate of around 5%. [73] [74]
CPM is diagnosed when some trisomic cells are detected on chorionic villus sampling and only normal cells are found on a subsequent prenatal test, such as amniocentesis or fetal blood sampling. In theory, CPM is when the trisomic cells are found only in the placenta.
Testing Women, Testing the Fetus by Rayna Rapp is a book, published in 1999, about analysis of the social repercussions of prenatal genetic testing.Rapp combines the data she collected herself with historical context of amniocentesis and genetic counseling to argue that amniocentesis and those abortions following positive test results is a social decision as much as an individual one.