NORTHWEST ARKANSAS PRENATAL GENETIC SCREENING
2nd trimester screen
Second trimester screening is a screening test which shows if you are at an increased risk of having a baby with Down syndrome, Trisomy 18, or an open neural tube defect, such as spina bifida. It requires a sample of your blood during the early second trimester.
Reasons for Testing:
Your reasons for testing may include:
concerns regarding Down syndrome and trisomy 18
concerns regarding open neural tube defects in pregnancy, such as spina bifida
a family history of Down syndrome, trisomy 18, or open neural tube defects
Testing for second trimester screening screens for:
Down syndrome and trisomy 18.
Open neural tube defects, such as spina bifida and anencephaly.
What is Down syndrome?
Down syndrome is caused by the presence of an extra chromosome (#21) and results in both mental and physical abnormalities. Approximately 1 in 800 babies is born with Down syndrome. The risk of having a child with Down syndrome gradually increases with age of the mother, but can occur at any maternal age.
What is trisomy 18?
Trisomy 18 is caused by the presence of an extra chromosome (#18) and results in serious mental retardation and physical deformities, including major heart defects. Approximately 1 in 6500 babies is born with trisomy 18. Only 1 out of 10 babies affected with trisomy 18 lives past the first year of life. As with Down syndrome, the risk of having an affected child gradually increases with the age of the mother.
What are open neural tube defects?
The neural tube, which forms very early in pregnancy, eventually develops into the baby’s brain and spinal cord. If this tube does not close completely, an opening remains along part of the baby’s spine or head. This can lead to paralysis and other physical and/or mental problems. Open neural tube defects occur in about 1 out of every 1500 live births. The risk of having a child with an open neural tube defect does not increase with the age of the mother.
How the test works:
Between approximately 15 and 21 weeks of your pregnancy (the optimal time is between 16 and 18 weeks), a small amount of your blood is drawn and certain proteins and hormones are measured. These proteins and hormones are made by your body, the developing baby and the placenta during pregnancy, and are found in every pregnant woman’s blood. However, when a fetus is at risk for Down syndrome, trisomy 18 or an open neural tube defect, the amount of these proteins may be abnormal.
What's not detected?
Second trimester screening does not screen for chromosome abnormalities or birth defects other than those mentioned above.
A screening test does not provide a diagnosis; it predicts the likelihood of a problem to occur. Second trimester screening tells you if there is an increased risk of your baby having Down syndrome, trisomy 18 or an open neural tube defect.
Second trimester screening leads to the detection of approximately:
81% of Down syndrome cases
80% of trisomy 18 cases
80% of open neural tube defects
The result “screen negative” means there is a low risk of your baby having Down syndrome, trisomy 18 or an open neural tube defect. However, in approximately 20% of pregnancies, these birth defects will not be detected through screening.
The result “screen positive” does not necessarily mean that your baby has one of these birth defects, but does mean that there is an increased risk for them. The next step is to talk with your doctor and/or genetic counselor about your results and your diagnostic testing options.
Some factors influencing your result:
exactly how far along you are in your pregnancy when the blood sample is taken
your weight, ethnic background and age
whether you are an insulin-dependent diabetic or take certain types of medications whether you have had a prior pregnancy or a close relative affected with Down syndrome, trisomy 18, or an open neural tube defect
Screening helps doctors to identify women who should be offered additional testing. Your doctor may recommend:
a more detailed ultrasound examination (which can often identify an open neural tube defect)
diagnostic testing, such as amniocentesis for Down syndrome, trisomy 18, and open neural tube defects