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