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Prenatal Testing-Amniotic Fluid

BACKGROUND

Amniocentesis is performed at the 15th to 17th gestational week, and sometimes as early as the 11 th to 14 th week. Testing up to 22 weeks may allow time for diagnosis and pregnancy termination in case of an abnormality. Third trimester chromosomal studies are used to confirm suspected defects prior to delivery and to aid in medical management.

INDICATIONS FOR TESTING

  • Advanced maternal age (35 years or older at time of delivery)
  • Abnormal Quad Screen
  • Known chromosomal abnormality in the parent (translocation etc.)
  • Family history of malformative/mental retardation disorders
  • Family history of disease/syndrome which could be detected by molecular or biochemical methods (see molecular outline)
  • Previous child with chromosomal abnormality

SAMPLE REQUIREMENTS

Amniotic fluid is withdrawn through a stiletted needle not larger than 20 gauge by the Obstetrician under ultrasound guidance. Total volume of fluid should be between 20 to 30 ml. A smaller amount is accepted in early amniocentesis. Specimens with smaller than 5 mls of fluid are unacceptable. The first several ml of fluid should be discarded to decrease the chance of maternal cell contamination. After collection, place the specimen immediately into sterile capped 15 ml centrifuge tubes. Brown centrifuge tubes are incorrect for specimen collection. The specimen can be stored at room temperature overnight. Specimens that are excessively bloody, contain myconia, or are drawn later than 25 weeks of gestation may grow poorly. In addition, bloody amniotic fluid may yield false results due to maternal contamination.

SPECIMEN HANDLING

Amniotic fluid may be stored overnight in sterile centrifuge tubes at room temperature, or shipped in a Styrofoam container by overnight courier. Amniotic fluid drawn more than 3 days previously, frozen, or less than 5ml in amount is not acceptable. Amniotic fluid can be delivered to the laboratory Monday-Friday during regular working hours.

PROCEDURE

Chromosome analysis is performed on amniotic fluid. Twenty trypsin-Wright banded metaphase cells (or 15 colonies) are counted and analyzed under the microscope. Two karyotypes are prepared at a minimum level of 450 bands. The 450 band level is recommended for prenatal diagnosis. Cells with extra or missing chromosomes can be detected, as well as chromosomes rearrangements involving several bands. Minute chromosomal rearrangements cannot be visualized at this level. Counting 20 cells (15 colonies) will detect mosaicism greater than 14% with 95% confidence. Additional counts or banding techniques may be performed at our discretion. Parental blood may be required in some cases of prenatal diagnosis for clarification of chromosomal polymorphisms.

Twenty cells and two karyotypes are prepared.

INTERPRETATION

Amniotic fluid will detect fetal sex, and any gross abnormalities of the sex chromosomes or autosomes such as trisomies, monosomies, translocations, deletions and duplications. Deletions and rearrangements on the molecular level cannot be seen.

RESULTS

Results are presented according to the international standards for chromosomal nomenclature (ISCN), and a full explanation of the karyotype is provided. Any abnormal or variant results are followed by genetic counseling.

Molecular, biochemical and FISH testing can also be done on fresh or cultured cells. Alpha-fetoprotein testing can be performed for detection of neural tube defects.