Handbook of Genetic Counseling/First Trimester Screening

First Trimester Screening Overview Screen low-risk population to identify those at higher risk to give birth to child with a chromosome abnormality Normal result does not mean that there are no abnormalities Abnormal result indicates that diagnostic testing by amniocentesis or CVS is warranted Consists of ultrasound and blood test Performed between 10th and 14th week of pregnancy Procedure Biochemical screening Blood draw done by finger stick onto filter paper May be taken between 8 weeks, 4 days and 13 weeks, 3 days Measures levels of substances found in the blood of all pregnant women FreeBeta Human chorionic gonadotropin (hCG) measurements Glycoprotein derived from the placenta Serum levels tend to be high in patients carrying a fetus affected with Down Syndrome (average = 1.9 MoM) Serum levels significantly reduced in patients carrying a fetus affected with Trisomy 18 (average = 0.18 MoM) Pregnancy Associated Plasma Protein A (PAPP-A) Produced by trophoblast of placenta Significantly reduced in blood of women carrying fetuses with Down Syndrome (average = 0.44 MoM) Significantly reduced in blood of women carrying fetuses with Trisomy 18 (average = 0.32 MoM) Ultrasonagraphic findings Measure nuchal translucency (NT) Amount of fluid that accumulates behind the neck of the baby Echo-free space between skin and soft tissue overlying cervical spine Present only during first trimester About 30-86% of fetuses with increased nuchal translucency have chromosomal abnormality Increased in fetuses with chromosome abnormalities, cardiac defects, and certain genetic syndromes Greater than 3.5 mm is considered out of normal range Increased NT is not diagnostic of chromosome abnormality Lymphedema seen in NT may be result of fetal cardiac abnormalities Usually resolves in second trimester No consensus on how to measure nuchal translucency resulting in conflicting results about effectiveness Measured transbdominally in 95% of cases and transvaginally in other 5% Procedure takes between 15 and 30 minutes Detection Rate Biochemical screening without NT Identifies greater than 60% of Down Syndrome Identifies 90% of Trisomy 18 About 1 in 25 women with an increased risk based on screening will have baby with Down Syndrome Biochemical screening with NT Identifies about 90% of Down Syndrome Identifies up to 97% of Trisomy 18 False positive rate is 5% About 1 in 17 women with an increased risk based on screening will have baby with Down Syndrome Cannot screen for open neural tube defects so AFP testing still recommended in 2nd trimester Cannot identify all birth defects Benefits and Limitations of Testing Benefits Helps reduce maternal anxiety at earlier gestational age Allows patient to have CVS for diagnosis in 1st trimester Allows for 1st trimester termination of affected fetus Not an invasive procedure - no risk to mother or to fetus Limitations Currently not covered by insurance Only screens for two out of possible thousands of genetic conditions Screening will miss about 10% of Down Syndrome Still requires invasive procedure for diagnosis Not as sensitive if multiple pregnancies Patient specific risks rely on accuracy of clinical information that is interpreted for results of test Research Study at Magee-Womens Hospital Eligibility criteria: women 18-45 between 10 and 14 weeks To determine if first trimester screening is an effective method to detect Down Syndrome and other chromosome abnormalities If risk higher than 1 in 270, can continue routine prenatal care, undergo CVS at 10-13 weeks, or have amniocentesis after 15 weeks Women who choose to participate must provide information on pregnancy outcome Responsible for all costs, including blood sample, ultrasound, and genetic counseling References "ACOG Committee Opinion: First Trimester Screening for Fetal Anomalies with Nuchal Translucency." American College of Obstetrics and Gynecology. http://www.medem.com "NIH News Release: First Trimester Screening for Down Syndrome Possible, NICHD-Funded Study Finds." National Institutes of Health. http;//www.nih.gov "Screening for Down Syndrome, Trisomy 18 Syndrome, and Open Neural Tube Defects in the First Trimester." GeneCare Medical Genetics Center". http://www.genecare.com Spencer K. "Accuracy of Down Syndrome Risks Produced in a First-Trimester Screening Programme Incorporating Fetal Nuchal Translucency Thickness and Maternal Serum Biochemistry." Prenatal Diagnosis (2002) 22;244-246. Notes The information in this outline was last updated in June 2002.