Prenatal screening for fetal aneuploidy.

OBJECTIVE: To develop a Canadian consensus document with recommendations on maternal screening for fetal aneuploidy(e.g., Down syndrome and trisomy 18) in pregnancy.

OPTIONS: Pregnancy screening for fetal aneuploidy started in the mid 1960s, using maternal age as the screening test. New developments in maternal serum and ultrasound screening have made it possible to offer all pregnant patients a non-invasive screening test to assess their risk of having a fetus with Down syndrome or trisomy 18 to determine whether invasive prenatal diagnosis tests are necessary. This document will review the options available for non-invasive screening and make recommendations for Canadian patients and health care workers.

OUTCOMES: To offer non-invasive screening for Down syndrome or trisomy 18 to all pregnant women. Invasive prenatal diagnosis would be limited to women who screen above a set risk cut-off level on non-invasive screening or pregnant women who will be 40 years at time of delivery who, after counselling, chose to go directly to amniocentesis/chorionic villi sampling (CVS). Currently available non-invasive screening options include maternal age combined with (1) first trimester screening (FTS) (nuchal translucency,maternal serum biochemical markers); (2) second trimester serum screening; or (3) two-step integrated screening, which includes first and second trimester serum screening with or without nuchal translucency (IPS, Serum IPS, contingent and sequential). These options are reviewed and recommendations are made.

EVIDENCE: A MEDLINE search was carried out to identify papers related to this topic that were published between 1982 and 2006. Practices across Canada were surveyed. A consensus document was drafted and reviewed by committee members.

VALUES: The quality of evidence and classification of recommendations followed discussion and consensus by the combined committees of SOGC (Genetics, Diagnostic Imaging)and CCMG (Prenatal Diagnosis).

BENEFITS, HARMS, COSTS: These guidelines are intended to reduce the number of amniocenteses done when maternal age is the only indication. This will have the benefit of reducing the numbers of normal pregnancies lost because of complications of invasive procedures. Any screening test has an inherent false positive rate,which may result in undue anxiety. A detailed cost-benefit analysis of the implementation of this guideline has not been done, since this would require health surveillance and research and health resources not presently available; however, these factors need to be evaluated in a prospective approach by provincial and territorial initiatives.