Fetal Diagnostic Center Services in Burlington, VT
First Trimester Down Syndrome Screening
Using a combination of ultrasound findings and a blood test, women are able to obtain the risk that their fetus is affected with Down syndrome (trisomy 21) or Edward syndrome (trisomy 18). This test is available between 11 weeks 3 days and 13 weeks 6 days of pregnancy.
Midtrimester Evaluation for Anomalies
The risk in the general population that a fetus has a major structural birth defect is 2 --3%. While not all abnormalities can be identified by ultrasound, many can be. When a comprehensive examination of the fetus is normal, the risk of a major abnormality falls to about 1%. Additionally, approximately one-half of fetuses with Down syndrome will show a major or minor marker, so if the ultrasound examination reveals no markers, we may cut a woman's risk for Down syndrome in half.
In fetuses at an increased risk for congenital heart defects, fetal echocardiography is available to assess the fetal heart. These studies are generally performed between 20 - 22 weeks of pregnancy.
Doppler ultrasound is a special type of ultrasound that is used to assess the health of the fetus and placenta. Assessment with Doppler ultrasound is usually reserved for fetuses at risk for or already affected by poor growth. It is also used to evaluate fetuses at risk for developing anemia. Occasionally it is used earlier in pregnancy to screen women who are at a high risk of developing preeclampsia (a disorder of pregnancy involving high blood pressure).
Biophysical Profile (BPP)
The BPP is an ultrasound-based test that examines fetal behavior in utero. It is used in some pregnancies at increased risk of stillbirth and seeks to identify fetuses that are affected by chronic processes that would lead to a stillbirth without intervention.
Invasive Testing at The UVM Medical Center
Amniocentesis involves using a thin needle (generally thinner than what is used to draw blood) to withdraw a small amount of amniotic fluid from the womb for a variety of tests. Although it carries a risk of miscarriage of about 1/250 (0.4%), it remains the safest test available between 15--20 weeks to test the fetus for chromosome abnormalities.
Chorion Villus Sampling (CVS)
CVS is an early test for chromosome abnormalities. It is performed between 10 and 14 weeks and is essentially a biopsy of the placenta. While the risk of miscarriage is slightly higher than with amniocentesis (1/150 or 0.7%), it carries the advantage of providing early diagnoses for couples at risk.
Percutaneous Umbilical Blood Sampling (PUBS)
Although it is fortunately rarely needed, drawing the blood of the fetus prior to birth for testing and in utero transfusion of the anemic fetus is available.
Other therapies, such as the placement of bladder or chest shunts prior to birth, are available when necessary.
First Trimester Screening Program
For women and families who want early assessment of the risk that their pregnancy has Down syndrome, UVM Medical Center and the University of Vermont offer the First Trimester Screening. Traditionally, screening for Down syndrome has been performed in the second trimester, usually between the 15th and 20th week. Recently, progress has been made in identifying women at increased risk for Down syndrome between 11.5 and 14 weeks with ultrasound and blood tests.
Why consider first trimester screening?
The primary benefit is that we can offer an early assessment of the pregnancy. For some, early assessment is desired to offer early reassurance that the pregnancy is not affected by Down syndrome. For others, it is important to identify abnormalities in the pregnancy as soon as possible so that they are better able to manage their pregnancy.
What is involved in the test?
You will have a blood test and ultrasound performed together between 11 weeks 3 days and 13 weeks 6 days. These tests are used together with your age to provide an estimate of the risk that your baby has Down syndrome.
What abnormalities are screened for?
With ultrasound and the blood test, we can give you a risk assessment for Down syndrome. That means we can tell you the odds that your pregnancy is affected. We cannot tell you for sure, based on ultrasound and the blood test, that your pregnancy has Down syndrome. Additionally, there are a few structural abnormalities (birth defects) that can be seen on an early ultrasound. Measurement of the nuchal translucency (area behind the neck) has become a primary means of screening for chromosome abnormalities in the first trimester.
If I find out that I am at increased risk, how can I find out if I am affected or not?
There are 2 basic means of identifying chromosomal abnormalities, such as Down syndrome, in a pregnancy. The first is performed between 10 and 14 weeks and is called chorion villus sampling (CVS). It consists of taking a small biopsy of the placenta (afterbirth). This test carries a small chance (1%) of causing a miscarriage. Amniocentesis is also available from 12 weeks on and involves removing a small amount of amniotic fluid for testing. Prior to 15 weeks, it is referred to as an "early" amniocentesis and carries risks similar to CVS. From 15 weeks on, the risk that an amniocentesis will cause a miscarriage is between 1/200 and 1/250.
Are there drawbacks to early screening?
The main drawback is that in the second trimester, when screening has been most commonly done in the past, we are also able to look for many of the major birth defects that can affect a pregnancy. The number of birth defects we will suspect from a first trimester scan is limited. The other drawback is that second trimester serum screening (blood test) will also offer information on the risk of a neural tube defect (such as spina bifida) and can identify some pregnancies at risk for poor fetal growth during the pregnancy.
If I have first trimester screening, am I disqualified from second trimester screening?
We do not recommend having both first trimester screening and a second trimester quad marker performed. In a pre-screened population, the second trimester test will generally overestimate the true risk for Down syndrome. We do recommend having just the maternal serum alphafetoprotein (MSAFP) alone drawn to screen for spina bifida and poor fetal growth.
Insurance companies have traditionally covered the costs of second trimester screening for patients at increased risk for Down syndrome (such as those who will be more than 34 years old at delivery or those with abnormal multiple marker screens). First trimester screening is relatively new, and other than in the case of women with a prior child with Down syndrome, it is not yet clear just what will and won't be routinely covered. We are not aware of anyone having coverage problems, but you may wish to check with your insurer prior to requesting entry into the first trimester screening program.