For decades, researchers have been trying to sort out tests that can be done short of amniocentesis or chorionic villus sampling (CVS) that permit us to identify pregnancies at increased risk for chromosome problems. The current testing recommended by the state of California includes the following:
- An ultrasound examination at 11 to 14 weeks (embryo measuring 45 to 85 mm) including nuchal translucency measurement. It is possible to calculate Down syndrome risk, for example, simply from mom’s age and a nuchal translucency measurement. However, this test is not particularly accurate. Even if we flag 10 percent of all pregnancies as being at increased risk based upon the nuchal translucency measurement and Mom’s age, and test all of these pregnancies, we will only identify about 75 percent of Down syndrome pregnancies.
- The state of California has used biochemical screening between 15 and 20 weeks for many years. Currently, the state program uses four chemicals detected in mom’s bloodstream – beta hCG, maternal serum AFP, estriol, and inhibin. A Down syndrome risk calculation can be done incorporating these measurements and mother’s age. In the past, by flagging about 6 percent of all pregnancies as being at increased risk for Down syndrome, the state program had a detection rate of about 75 percent.
- More recently, research has shown that an integrated screening, using the nuchal translucency measurement, the quad screen, and also a first trimester blood test, can be used to calculate Down syndrome risk more accurately. Women who have a blood sample drawn between 10 and 13 weeks for hCG and PAPP-A, and then have a nuchal translucency screen, can be given a preliminary Down syndrome risk calculation. About 2.5 percent of all women will show an abnormal first trimester screen. If we add the quad screen at 15 to 20 weeks, then the Program will flag about 5 percent of pregnancies as being at increased risk. Among women with an abnormal quad screen, we identify 85 to 90 percent of Down syndrome pregnancies, and about 80 to 85 percent of trisomy 18 pregnancies.
What if mom does not have the first trimester lab work done?
The state program can still calculate risk for Down syndrome by means of the nuchal translucency screening, plus the quad screen. This has two disadvantages. First, it is a little less accurate – the detection rate is about 5 percent lower. Second, there is no calculation until after the quad screen has been done, so that detection occurs later.
What if it is not possible to measure a nuchal translucency?
Studies have shown that it will not be possible to measure the nuchal translucency with sufficient accuracy to be used in Down syndrome risk calculation in between 5 percent and 15 percent of nuchal translucency scans. Among these women, a risk calculation for Down syndrome can be made based upon the first trimester lab work and the quad screen. The results will be slightly less accurate – about 5 percent lower detection rate for Down syndrome.
What if my Down syndrome screening test comes back positive?
The California Prenatal Screening Program (also known as the AFP Program) will contact your physician with the results. The fee for the lab test includes follow-up services, including genetic counseling, and, if mom so desires, a detailed sonogram and either CVS or amniocentesis testing. These services must be performed in a state-approved prenatal diagnostic center, such as San Diego Perinatal Center.
If the screening results come back positive, then you will meet with a genetic counselor, discuss the results and the risk calculation, and discuss options of how to proceed. If the result comes back early (before 13 weeks), then the options will include amniocentesis or CVS test. If the results come back later on, then you will have a detailed ultrasound scan, and, if you wish, an amniocentesis test.
The state’s Prenatal Screening Program lab work also will provide a risk calculation for Down syndrome and trisomy 18, and also will provide a risk calculation for spinal cord and certain brain abnormalities (including anencephaly and spina bifida). About 95 percent of pregnancies affected by anencephaly (absence of the skull and most of the brain) will show an abnormal screening test, and about 85 percent of pregnancies affected by spina bifida will show an abnormal screening test.
Anencephaly can be diagnosed on ultrasound as early as 12 weeks, and with virtually 100 percent accuracy by 15 weeks. Spina bifida results in abnormalities visible on scan in more than 98 percent of cases after 15 or 16 weeks into the pregnancy.
If the screening test comes back abnormal, does that mean the baby has an abnormality?
No! The screening test is just that – a means of calculating risk for certain conditions of the baby. The screening test will give you a calculated risk for your baby to have a particular problem. For example, your calculated risk for Down syndrome might come back at 1 in 100, or 1 percent. This means that there is a 99 per 100 or 99 percent chance that your baby does not have Down syndrome. Each family will have the option of deciding whether the risk calculation is high enough that they wish to pursue diagnostic testing.