Canadian Guidelines for Prenatal Diagnosis - Genetic indications for prenatal diagnosis

Objective: To provide family practitioners, obstetricians, and geneticists with guidelines and recommendations for prenatal diagnosis. Options: These guidelines apply to non-invasive screening techniques (including maternal serum screening and ultrasound) and to invasive techniques (including amniocentesis and chorionic villus sampling). Outcomes: Improved prenatal diagnosis of congenital abnormalities, chromosomal anomalies or genetic conditions, and adverse outcomes related to prenatal testing procedures including pregnancy loss. Evidence: The English language medical literature between 1976 and 2000 was reviewed, and opinions were obtained from experts in prenatal diagnosis. The level of evidence for the recommendations was determined using the criteria described by the Canadian Task Force on the Periodic Health Examination. Benefits, harms, and costs: These guidelines will provide practitioners with a better understanding of the indications for prenatal diagnosis and the risks and limitations of available procedures. Recommendations: Maternal age should be used to determine which women are at increased risk of having a child with a chromosomal anomaly. (II-2 A) Screening tests such as maternal serum screening could be used to modify a woman’s agerelated risks. (II-2 A) Amniocentesis should be offered to women at increased risk. (I A) Chorionic villus sampling can be offered as an alternative to amniocentesis. (I A) Validation: These guidelines update the 1993 “Canadian Guidelines for Prenatal Diagnosis of Genetic Disorders.” Recommendations were reviewed and revised by the Prenatal Diagnosis Committee of the Canadian College of Medical Geneticists and the Genetics Committee of the Society of Obstetricians and Gynaecologists of Canada (SOGC), and were approved by the SOGC Council. Sponsors: The Canadian College of Medical Geneticists and the Society of Obstetricians and Gynaecologists of Canada. INCREASED RISK FOR CHROMOSOME ABNORMALITIES In the Canadian health care system, invasive prenatal genetic testing is not generally offered to all women. Guidelines for access to testing are an attempt to balance genetic risks against procedural risks and economic considerations. Several screening techniques are currently employed to determine whether a couple is at increased risk to have a child with a chromosomal imbalance.1 The current standard for chromosome testing involves cell culture and evaluation of all chromosomes by a banding method (usually G banding) after an invasive procedure such as amniocentesis, chorionic villus sampling (CVS) or fetal blood sampling. In addition, based on the clinical indication for testing, syndrome-specific fluorescence in situ hybridization (FISH) probes for microdeletion detection (as in velocardiofacial/DiGeorge) may be used. Interphase FISH testing for common trisomies and sex chromosome abnormalities may be appropriate to provide a rapid response when the patient is referred relatively late (after 21 weeks) or multiple congenital abnormalities are identified by ultrasound. In this circumstance, the patient and physician must be made aware of the limitations of interphase testing and that structural chromosome abnormalities and rare trisomies will not be detected.2 Prenatal tests can be divided into two categories: screening tests and diagnostic tests. Examples of screening tests for chromosome abnormalities include asking a woman’s age, maternal serum screening, and ultrasound examination. Diagnostic testing includes CVS, amniocentesis, and cordocentesis.