Dipl.-Biol. Uwe Heinrich
The major invasive prenatal diagnostic procedures are chorionic villus sampling (CVS), amniocentesis (AC) and umbilical cord puncture (cordocentesis). The method is chosen according to the clinical situation. CVS, during which 10-30 mg of chorionic villi is biopsied, is usually done between the 11th and 13th week of pregnancy. Both a direct preparation as well as a long-term culture is prepared from this tissue. The direct preparation provides a preliminary result on the following day, while the final result is available once the two-week long-term culture has been analyzed. In experienced medical centers, the risk of miscarriage after CVS is at 0.5-1%. Early amniocentesis is done within the 13th-15th week of pregnacy, or as classic AC within the 15th-17th week. Under ultrasound guidance, 10-20 ml of amniotic fluid is taken transabdominally with a thin needle. From the cells of the amniotic fluid, several long-term cultures are prepared. Results are available after approximately 2 weeks. The risk of miscarriage is between 0.3 and 0.5%. Cordocentesis is possible from approx. 20 weeks; the result is available within one week. The risk of miscarriage is 0.5-1%. If a monogenic disorder (e.g. osteogenesis imperfecta) is either present or suspected, genomic DNA can be extracted for molecular genetic testing from all above-mentioned samples as well. Contaminations with maternal cells can be ruled out by a microsatellite analysis. An ETDA blood sample from the mother is necessary for this procedure.