TAILIEUCHUNG - Chapter 063. Chromosome Disorders (Part 4)

Cytogenetic Testing in Prenatal Diagnosis The vast majority of prenatal diagnostic studies are performed to rule out a chromosomal abnormality, but cells may also be propagated for biochemical studies or molecular analyses of DNA. Three procedures are used to obtain samples for prenatal diagnosis: amniocentesis, chorionic villus sampling (CVS), and fetal blood sampling. Amniocentesis is the most common procedure and is routinely performed at 15–17 weeks of gestation. On some occasions, early amniocentesis at 12–14 weeks is performed to expedite results, although less fluid is obtained at this time. Early amniocentesis carries a greater risk of spontaneous abortion or fetal. | Chapter 063. Chromosome Disorders Part 4 Cytogenetic Testing in Prenatal Diagnosis The vast majority of prenatal diagnostic studies are performed to rule out a chromosomal abnormality but cells may also be propagated for biochemical studies or molecular analyses of DNA. Three procedures are used to obtain samples for prenatal diagnosis amniocentesis chorionic villus sampling CVS and fetal blood sampling. Amniocentesis is the most common procedure and is routinely performed at 15-17 weeks of gestation. On some occasions early amniocentesis at 12-14 weeks is performed to expedite results although less fluid is obtained at this time. Early amniocentesis carries a greater risk of spontaneous abortion or fetal injury but provides results at an earlier stage of pregnancy. The vast majority of amniocenteses are performed in the context of advanced maternal age the best-known correlate of trisomy see below . Additional reasons for amniocentesis referral include an abnormal triple- or quad marker assay and or detection of ultrasound abnormalities. In this assay levels of human chorionic gonadotropin a-fetoprotein and unconjugated estriol and in the quad assay inhibin in the maternal serum are quantified and used to adjust the maternal age-predicted risk of a trisomy 21 or trisomy 18 fetus. Specific ultrasound abnormalities when detected at midtrimester can also be associated with chromosomal defects. When a nonspecific ultrasound abnormality is present the estimated risk of a chromosomal defect is 16 . Associations of chromosomal abnormalities and specific types of abnormal ultrasound findings are listed in Table 63-1. Table 63-1 Frequency of Chromosome Abnormalities Identified on the Basis of Abnormal Ultrasound Findings Ultrasound Finding Chromosomal Abnormalities Frequency Average Range in Different Studies Abnormal ultrasound nonspecific 16 13-35 Omphalocele 39 26-54 Cystic hygroma 68 46-78 Congenital heart disease 30 8-40 Choroid plexus cyst 5 4-10 CVS is the second .

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