Rapid prenatal diagnosis The Genetics Center, Inc.


	Rapid Prenatal Chromosome Diagnosis: Straightforward answers to important questions

Rapid prenatal chromosome diagnosis is very useful in selected circumstances. Unlike conventional chromosome analysis on an amniocentesis or chorionic villus sample, which can take up to 14 days because the procedure entails culturing cells, this technique enables identification of Down syndrome and certain other chromosome abnormalities in 24 to 72 hours.

What is rapid prenatal diagnosis?

Fluorescent DNA probes are used to analyze the chromosome makeup of cells obtained from amniotic fluid or a chorionic villus sample (CVS). While not all chromosome abnormalities can be identified by this technique, the majority of common abnormalities can be identified. These include Down syndrome (trisomy 21), trisomy 18, trisomy 13, Klinefelter syndrome (47,XXY), triple-X syndrome (47,XXX), Turner syndrome (45,X) and 47,XYY. The technique is known as interphase FISH, which stands for fluorescent in situ hybridization, or simply as FISH. Our laboratory uses the AneuVysion test, which has been cleared by the FDA for this purpose.

When should this be considered?

Sometimes important pregnancy management decisions need to be made quickly, while waiting for a complete cytogenetic result. The following are some of the situations in which a rapid assay may be helpful.

Maternal age, serum screening, or ultrasound findings indicates a high risk for an Down syndrome or trisomy 18.

Prenatal chromosome diagnosis is indicated, but the patient is more than 20 weeks gestation.

An ultrasound examination in the second or third trimester suggests the fetus has Down syndrome, trisomy 13, or trisomy 18.

A patient has concern about whether the fetus has a chromosome abnormality; for example, a chromosome problem was discovered in a previous pregnancy.

What are limitations of rapid prenatal diagnosis?

This analysis does not screen for all chromosome abnormalities, only missing or extra copies of chromosomes 21, 18, 13, X, and Y.

Prenatal FISH is not designed to detect chromosome mosaicism, partial chromosome duplications and deletions, or structural rearrangements.

Although prenatal FISH analysis is highly accurate, irreversible therapeutic decisions should not be made on FISH results alone.

An additional 5-10ml of amniotic fluid is required for the rapid FISH assay, in addition to the 15-20ml required for conventional chromosome studies. Samples with visible blood may not be suitable for FISH analysis.

Genetic consultation for patients undergoing prenatal diagnosis, including rapid prenatal diagnosis, is often very helpful. This can help patients understand limitations of prenatal diagnosis and FISH analysis and enables physicians to order the optimal combination of prenatal diagnostic analyses.