Understanding Your Genetic Testing Options: Screening
What It Is:
The triple screen is so named because it is a trio of maternal blood tests that are performed to identify problems in the fetus, especially Down syndrome. The three tests are MSAFP, beta-human chorionic gonadotropin, and unconjugated estriol. Each of these proteins are made and utilized by your unborn baby. Babies that have Down syndrome or other chromosome abnormalities make and utilize them differently. The differences in maternal blood levels help your doctor assess the risk that your baby has Down syndrome or other chromosomal problems.
When It Is Performed:
This blood test is usually administered between the fifteenth and twentieth week of pregnancy.
The three combined tests alert doctors to about 65 percent of the women carrying an infant with Down syndrome. Unfortunately, the false positive rate is four to five percent. Diabetes, multiple gestation (twins), race, and in vitro fertilization, in addition to Down syndrome, all influence the triple screen values. A limitation to the triple test is that it misses some fetuses with Down syndrome. More accurate maternal blood tests for identifying Down syndrome should be available soon. For now these blood tests are only risk-assessment tools. Only invasive testing can diagnose or exclude chromosome problems.
What It Is:
Ultrasound takes pictures of the fetus, uterus, and placenta. The ultrasound is performed for many reasons. It can be used to estimate your due date, monitor fetal growth, and screen for problems in the fetus or mother.
When It Is Performed:
The accuracy of fetal assessment by ultrasound varies according to the duration of gestation at the time of exam, quality of the equipment used, skill of the operator, extent of the malformation, and length of time spent looking for abnormalities. Therefore, it is possible to have one or more ultrasounds performed primarily to monitor fetal growth or estimate the due date and NOT see a malformation in the fetus. If screening blood tests or a screening ultrasound suggest a problem, a more detailed ultrasound evaluation for fetal problems may be necessary at a referral center.
If a serious birth defect is present, it will probably change the care you and your baby receive. For example, MSAFP is good at identifying babies with spina bifida (about 85 percent). If your baby has this problem, you should have a C-section since vaginal deliveries appear to increase the paralysis associated with spina bifida. MSAFP can also identify some babies with Down syndrome, abdominal wall defects, placental problems, and other situations that may require special care for you or your infant. In some cases, you will have to change the hospital where you plan to deliver so that your infant will be born near pediatric specialists.
Screening tests help you to know if there is a problem with your fetus or with your pregnancy. Knowing about problems before delivery influences many decisions. For example, newborns with complicated problems have better outcomes when delivered at a hospital with a newborn intensive care unit (NICU). Knowing that your baby will have a problem permits preparation to provide your baby just what is needed from the moment of birth. Some falsely positive screening tests occur. These mean that you were identified as having an increased risk of a problem even though your baby was OK. Overall screening tests can help you and your unborn baby.
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