# 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.