Amnio doesn't tell whole story with CNS abnormalities. (Family history, MRI also important).

SAN FRANCISCO -- One of the most useful rules about genetic counseling for CNS abnormalities is this: Amniocentesis doesn't tell you everything.

That advice was offered by Dr. Mary Norton at a meeting on antepartum and intrapartum management sponsored by the University of California, San Francisco.

Often more detailed testing, including fetal MRIs and genetic counseling, is needed for families with fetal CNS abnormalities. "The family history and pedigree, for instance, are critically important in making a diagnosis," said Dr. Norton, director of the Prenatal Diagnosis Center at the university.

Other important rules to keep in mind in genetic counseling are that the presence of a second anomaly greatly impacts the outlook for the fetus, and that the prognosis for a fetus is often not the same as for a neonate.

One example is bilateral clubfoot. In an infant, clubfoot is often not a big problem. "The infant may just need surgery or casting and can live a completely normal life," Dr. Norton said. But clubfoot in a fetus is frequently a marker for other chromosomal abnormalities, such as trisomy 18, which has a 60%-70% stillborn rate.

Although amniocentesis informs the obstetrician about chromosomal abnormalities, it shouldn't be overly relied upon. "It doesn't tell you much about categories of genetic disease that don't stem from chromosomal abnormalities," Dr. Norton said.

Amniocentesis detects chromosomal abnormalities with close to 100% accuracy and it can diagnose other genetic disease if the DNA or biochemical basis is known. But a routine amniocentesis cannot tell you about genetic problems, such as autosomal dominant or recessive disease or X-linked or congenital malformations.