The Fetal Center at Children's Memorial Hermann Hospital offers a full array of prenatal screening and diagnostic testing. First Trimester Screening is available to all pregnant women. Traditionally, diagnostic testing has been offered to all women with an abnormal screening test during pregnancy as well as women who will be 35 years of age or older at delivery.
Currently, the American College of Obstetricians and Gynecologists recommends that all pregnant women regardless of risk factors should have the option of making an informed decision about diagnostic testing. Non-invasive screening tests are available in the first (nuchal translucency/biochemistry) and second trimester (quadruple screening).
Additionally, there are three diagnostic (invasive) testing options: chorionic villus sampling, amniocentesis, and fetal blood sampling.
These screening tests may slightly increase the detection rate and decrease the false positive rate in comparison to FTS. If the baby measures too large for FTS, or if a nuchal measurement cannot be obtained, you will be given the option of quadruple marker screening at your physician's office in the second trimester.
First Trimester Screening
While most babies are born healthy, 3-5% will have birth defects. The Center provides many screening and testing options during pregnancy that are available to provide more information about the risk of having a baby with certain types of birth defects. Down syndrome and trisomy 18 are two abnormalities that are caused by an extra chromosome.
What is First Trimester Screening?
First Trimester Screening (FTS) is a screening test designed to identify pregnancies at higher risk for Down syndrome and trisomy 18 in the first trimester. FTS does not diagnose either condition, but provides an estimate of risk, or chance, that the baby has the condition. Even with a positive screen, most babies do not have abnormal chromosomes.
When is FTS performed?
This is an optional test that can be performed between the beginning of the 11th and end of the 13th week of pregnancy.
How is FTS performed?
FTS combines three factors:
- Mother's age: Every woman has some risk to have a child with a chromosome abnormality like Down syndrome or trisomy 18; however, the risk increases as the mother's age increases.
- Blood sample: A sample of mother's blood (fingerstick) is analyzed for naturally occurring proteins and hormones that are made by the baby and the baby's placenta. Different levels of these proteins and hormones have been associated with an increased risk for Down syndrome and/or trisomy 18.
- Ultrasound examination: Ultrasound can be used to look for characteristics of the fetus that may suggest chromosomal problems. The crown-rump length will be measured to make sure that you are in the right week of pregnancy for the test. Then a measurement will be taken of the thin layer of fluid found at the back of the baby's neck called the nuchal translucency (NT). An increased NT measurement is associated with an increased risk for Down syndrome, trisomy 18, and other chromosome abnormalities.
What do the results mean?
The results from FTS will give a specific numeric risk for both Down syndrome and trisomy 18. If the results show an increased risk for Down syndrome or trisomy 18, a genetic counselor from the Center will discuss the results with you and provide you with information on further diagnostic testing. Genetic counselors are health care professionals trained to discuss various genetic risk factors during pregnancy and review pregnancy testing options, including risks, limitations and benefits, of screening and diagnostic testing.
"A Positive Test" A positive result means that the pregnancy is at an increased risk for Down syndrome or trisomy 18. It does NOT mean that the baby has a chromosome problem. In this situation, genetic counseling will be offered to discuss the option of further diagnostic testing such as CVS or amniocentesis to help determine whether or not the baby has a chromosome abnormality.
"A Negative Test" A negative result means that the pregnancy is not at an increased risk for Down syndrome or trisomy 18. In the absence of other risk factors, further diagnostic testing is not needed. A negative FTS result does not eliminate the possibility of having a baby with Down syndrome or trisomy 18. It does not rule out all birth defects or genetic conditions.
FTS cannot determine if a baby has Down syndrome or trisomy 18, but can be used to help determine the risk for having these conditions. This gives expectant parents more information to help them decide whether or not to proceed with further testing. Approximately 80-90% of pregnancies with Down syndrome and trisomy 18 will screen positive on this test.
Amniocentesis is used in the prenatal diagnosis of chromosomal abnormalities and fetal infections, in which a small amount of amniotic fluid, which contains fetal cells, is collected from the amniotic sac surrounding a developing fetus, and the fetal DNA is examined for chromosomal abnormalities. A small needle is inserted through the mother's abdominal wall and uterus into the amniotic sac under direct ultrasound guidance. Approximately 20 mL of amniotic fluid are collected for testing. The puncture site usually seals over, and the amniotic fluid is replenished in a few hours. Under a microscope the chromosomes are examined for abnormalities. The most common abnormalities detected are Down syndrome, trisomy 18, trisomy 13, and Turner syndrome. Amniocentesis is most safely performed after the 15th week of pregnancy.
Chorionic Villus Sampling (CVS) is an invasive test performed in the first trimester that can determine chromosomal or genetic disorders in the fetus by obtaining a sample of the placental tissue, also known as the chorionic villi, to evaluate the baby's chromosomes. The advantage of CVS is that it can be carried out at 11-13 weeks of gestation at a time earlier than amniocentesis. CVS can be done through the cervix (transcervical, Figure 1) or through the abdomen (transabdominal, Figure 2).. An abdominal ultrasound is performed to determine the position of the uterus, the size of the gestational sac, and the position of the placenta within the uterus. The transcervical procedure is performed by inserting a thin plastic tube through the vagina and cervix to reach the placenta. Your doctor uses ultrasound images to help guide the tube into the appropriate area and then removes a small sample of chorionic villi. The transabdominal approach is performed by inserting a needle through the abdomen and uterus and into the placenta under direct ultrasound guidance.
Fetal blood sampling is performed by specialists to help diagnose, treat, and monitor fetal problems at various times during pregnancy. A fetal blood sample may be taken to diagnose chromosome abnormalities, assess and treat severe fetal anemia (low red blood cell level), check for fetal infections and give certain medications to the fetus. There are several ways blood can be obtained from the fetus. After cleansing the mother's abdomen with an antiseptic solution, a long, thin needle is inserted into the mother's uterus under ultrasound guidance. Blood may be taken from the umbilical vein or from a vein of the liver. If severe fetal anemia is diagnosed, a fetal blood transfusion can be performed.
Contact The Fetal Center to make an appointment with a high-risk pregnancy doctor.