Patient information on screening tests
There are several screening options for you to choose from. It can be very overwhelming to decide as to which, if any, is appropriate for you. Most of all, we want you to enjoy your pregnancy and look forward to the best outcome possible. Many people decide against any testing at all- their reasons may be varied from- “I wouldn’t do anything different anyways”, to “It would just make me worry the whole pregnancy”, to “I will deal with the issues once the baby is born”, to “I know there are a lot of false positives and I don’t want to be led down the wrong path”. There are no right or wrong decisions. You have to make the right decision for you. Hopefully, the information provided here will help you make that decision.
First trimester screening tests
First trimester screening tests offer several potential advantages over second trimester screening. When tests are negative (normal), it may help reduce maternal anxiety earlier. If tests are positive (abnormal), it allows women to take advantage of prenatal chorionic villus sampling (CVS) at 10-12 weeks, rather than the traditional amniocentesis usually done after 15 weeks. Detecting problems earlier in the pregnancy may allow women to prepare for a child with potential health problems. It also affords women greater privacy and less health risks if they elect to terminate the pregnancy.
Research has shown an association between fetuses with certain chromosomal abnormalities and ultrasonographic findings of an abnormally increased nuchal translucency (the area at the back of the fetal neck) between 11 1/7 to 13 6/7 weeks gestation. The best time is at 11 2/7 weeks. This test can help screen for Trisomy 21 (Down Syndrome), Trisomy 18 and also some babies with heart defects, such as atrial or ventricular septal defect, tetralogy of Fallot, coarctation of the aorta or aortic stenosis, hypoplastic left heart and multiple mixed cardiac anomalies.
In the first trimester, we can also measure proteins in your blood secreted from your baby. The 2 proteins we measure are PAPP-A and beta-HCG. Babies with Down syndrome make lower amounts of PAPP-A but higher amounts of HCG. Babies with trisomy 18 make decreased amount of these proteins. This test can be drawn the same time as your ultrasound.
The decision to proceed with any type of first trimester screening is recommended only for those people who would be willing to proceed with CVS or amniocentesis based on an abnormal test result. This CVS test is best performed between 10 and 12 6/7 weeks. CVS has a procedure related risk of pregnancy loss of 1/200 when performed by an experienced physician. Concerns about limb reduction defects associated with CVS have been raised in the past but have been found when CVS is performed before 9 weeks gestation. Therefore, we would not recommend CVS until well after the window where limb reduction defects have been reported. In addition in 1% of CVS samples the presence of an abnormal cell line that does not represent the fetus but is confined to the placenta is identified. In some of these cases amniocentesis is offered to confirm that the fetus is not affected.
There are different strategies for first trimester screening for Trisomy 21 (Down syndrome) and Trisomy 18.
1.First Trimester Combined
This is a blood test which includes serum PAPP-A and HCG as well as a simultaneous ultrasound to measure the nuchal translucency.
2.Stepwise Sequential Screening
This involves the tests in the “first trimester combined”. If the result is abnormal, CVS or amnio will be recommended. If the result is normal, a quad screen will be done at 15-17 weeks.
This involves the tests in the “first trimester combined”. The results are withheld to your and your provider. A quad screen is then performed at 15-17 weeks. The result is calculated with a statistical analysis of all components together. The advantage of this test is that it provides the lowest False Positive rate (being told the result is abnormal, but your baby is really not affected with Downs syndrome or Trisomy 18)
4. Sequential/integrated Screen
This is the test performed at diamond women’s center which is a hybrid of the sequential and integrated screen. If the first trimester results are abnormal, these are reported to you. The second trimester screening at 15-17 weeks then recalculates all of the components together. In addition, information about your risk of neural tube defects is also calculated.
If you decide on the “first trimester combined screening” you will still need a mid trimester maternal blood AFP test to screen for neural tube defects.
Second trimester screening
You have the option of choosing this maternal blood test called the QUAD SCREEN done at 15-18 weeks. You may chose to do this test without the prior first trimester screen if you wish.
REMEMBER: First trimester or second trimester screening with out confirmatory CVS or amniocentesis cannot exclude the possibility of a chromosomal abnormality, but only provides an adjusted risk score.
Screen (NT plus
|82% at 13 weeks|
85% at 12 weeks
87% at 11 weeks
|Sequential Screening||95%||Probable > 90%||80%|
|Integrated Screening||94-96%||Probable > 90%||90%|
|18-20 Week ultrasound ONLY||50-60%||80-90%||90%|
Non-invasive prenatal testing (NIPT)
This is the newest test available. It is a test which can be performed at 10 weeks gestation or later and can diagnose trisomy 13, 18, and 21. Blood is drawn from the mother. Fetal dna, which is found in maternal blood is analyzed. This can be combined with an early ultrasound to look for other anomalies as well. A second trimester quad screen should be done to look for neural tube defects. Because the nipt can have false positive results, (at the present time, this test is only offered to 1) women 35 years or older at time of delivery, 2) pregnacies with ultrasound findings suggestive of a baby with a trisomy, 3) women who have had a prior pregnancy affected with a trisomy, 4) a patient who has a current abnormal test for trisomy, such as the tests discussed above 5) women who have (or their partners have) a balanced robertsonian translocation with increased risk of trisomy 13 02 21.
Important points of genetic testing
Also, remember these screening tests cannot detect many other possible birth defects and cannot diagnose mental retardation.
It is important that you consider all options in a timely fashion so that we may perform these tests in the appropriate windows of time. If you have further questions, please talk with your health care provider.