- Screening tests give a probability (percent chance) that a fetus has a chromosomal problem. An increased probability may indicate the need for a diagnostic test.
- Diagnostic tests detect some chromosomal problems with certainty.
Ultrasound imaging has many uses. It can be performed as early as the fourth week after conception to document fetal cardiac activity. Ultrasounds are also used to verify the due date, look at fetal anatomy and monitor fetal growth.
Nuchal Translucency (NT)
Nuchal translucency is a screening test that assesses whether your baby is likely to have Down syndrome, trisomy 13, trisomy 18 or a congenital heart defect. An ultrasound is done when you are 11 ½-14 weeks pregnant. It measures the fluid under the skin at the back of the baby’s neck. All babies have some fluid. Many babies with Down syndrome have an increased amount of fluid.
If the NT shows an increased risk of your baby having a trisomy, diagnostic tests, such as CVS or amniocentesis, can be performed to provide a definitive answer. The NT and two blood tests taken in your obstetrician’s office make up the full integrated screening for Down syndrome, trisomy 13 or trisomy 18.
Amniocentesis is usually performed when you are 15-20 weeks pregnant. A sample of amniotic fluid from your womb is taken and examined in a laboratory. The test detects some chromosomal and genetic disorders in the fetus, including Down syndrome and spina bifida. However, amniocentesis does not detect every kind of abnormality. Results are available after 2-2 ½ weeks and are reported to you and your referring health care provider.
Chorionic Villus Sampling (CVS)
Chorionic villus sampling is usually performed when you are 10-12 ½ weeks pregnant. A sample of your placental tissue is taken and tested for chromosomal problems. In our team’s experienced hands, the risk with CVS is similar to amniocentesis. Results are available after 2-2 ½ weeks and are reported to you and your referring health care provider.
CVS does not provide information on neural tube defects such as spina bifida. For this reason, women who undergo CVS also need a follow-up blood test in their second trimester to screen for neural tube defects.
Doppler Blood Flow
Doppler blood flow is a non-invasive technique. It evaluates umbilical cord blood flow and detects inadequate placental function or abnormal changes in blood flow.
Nonstress Testing (NST)
Nonstress testing looks for changes in fetal heart rate to assess fetal well-being. NST is only available at Packard Children’s Hospital in Palo Alto.