Vol. 19: Spring 2001
Prenatal Diagnosis
Non-invasive Prenatal Diagnostic Techniques
Fetal Ultrasound
Medical sonography has benefited from rapid advances in technology, which provide better images at earlier gestational ages. Many women have an ultrasound examination in the second trimester, as a screen for structural malformations. There is increasing evidence that ultrasound markers can be used to adjust the risk that the fetus has Down syndrome. This is particularly true when the ultrasound findings are combined with the results of the multiple marker biochemical screen in maternal serum. It is anticipated that these noninvasive tools can be used to assess a patient's specific risk and to help her make informed decisions about whether to have amniocentesis.
Analogous to maternal serum marker screening, ultrasound identification of a fetal structural abnormality provides information to adjust the risk for a fetal chromosome abnormality. For example, identification of a fetal cardiac anomaly should lead to careful fetal evaluation for other malformations using targeted (formerly referred to as high-resolution) ultrasound. Fetal echocardiography may also be useful in delineating the cardiac defect. Amniocentesis is usually offered if a major fetal structural anomaly is present.
More controversial is how to evaluate less specific ultrasound findings, which have been associated with chromosome abnormalities such as Down syndrome. These include but are not limited to nuchal edema or translucency, hyperechoic bowel, pyelectasis, choroid plexus cysts, and short humerus and femur lengths. In some cases, the ability to assess the fetal markers is limited by the position of the fetus, the body habitus of the patient, or the gestational age.
As with any screening method, there will be false positives and false negatives. There does not appear to be a consensus about how to counsel a patient when the biochemical multiple marker screen and the evaluation of ultrasound markers give conflicting risk assessments.
Chromosome analysis of fetal cells remains the only definitive way to diagnose abnormal chromosome constitution prenatally. Patients who opt for targeted ultrasound and biochemical markers should be informed that normal screens might falsely reassure them. However, for patients who are having great difficulty deciding whether to proceed with amniocentesis, this may be a preferred first step. Also, for women unwilling to accept even a small risk of pregnancy loss from amniocentesis, this approach may provide some helpful information.
There is active research into the use biochemical markers and ultrasound markers in the first trimester to refine a patient's risk for carrying a fetus with Down syndrome. If this is proven to be reliable, it would have significant benefit to patients because of earlier diagnosis than the second trimester studies currently in use.
The Genetic Drift Newsletter is not copyrighted. Readers are free to duplicate all or parts of its contents. The Genetic Drift Newsletter is published semiannually by the Mountain States Regional Genetic Services Network for associates & those interested in Human Genetics. In accordance with accepted publication standards, we request acknowledgement in print of any article reproduced in another publication. The views expressed in the newsletter do not necessarily reflect local, state, or federal policy. For additional information, contact Carol Clericuzio, M.D., Editor, Department of Pediatrics, The University of New Mexico, Albuquerque, NM, 87131
Prenatal Diagnosis - Table of Contents
Introduction
Prenatal Diagnosis of Single Gene Disorders
Down Syndrome Maternal Serum Screening
Fetal Ultrasound
Amniocentesis
Chorionic Villus Sampling
Percutaneous Umbilical Blood Sampling
Preimplantation Genetic Diagnosis
The Future of Prenatal Diagnosis
TABLE: Timetable for Prenatal Diagnostic/Screening Procedures
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