Article Sections
Definition
First trimester screening is a prenatal test that offers early information about a baby's health. Typically, first trimester screening is done between weeks 11 and 14 of pregnancy — earlier than many other prenatal tests.
First trimester screening includes two steps:
- A blood test to measure levels of two pregnancy-specific substances
- An ultrasound exam to measure the size of the clear space in the tissue at the back of a baby's neck (nuchal translucency)
Using your age and the results of the blood test and the ultrasound, your health care provider can gauge your risk of carrying a baby who has Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18). Down syndrome causes mental retardation. Edwards syndrome causes more severe retardation and is often fatal by age 1.
If your risk level is low, first trimester screening can offer reassurance of a healthy pregnancy. If your risk level is high enough to cause concern, you may choose to follow first trimester screening with diagnostic testing — such as chorionic villus sampling or amniocentesis. These tests are more invasive but can reliably determine whether a baby actually has Down syndrome or Edwards syndrome.
Why it's done
First trimester screening is done to evaluate your risk of carrying a baby who has Down syndrome. The test also provides information about the risk of Edwards syndrome. The test may help detect certain heart defects or skeletal problems as well. First trimester screening doesn't evaluate the risk of neural tube defects, such as spina bifida.
Because first trimester screening can be done earlier than most other prenatal screening tests, you'll have the results early in your pregnancy. This will give you more time to make decisions about further diagnostic tests, medical treatment and the course of the pregnancy. If your baby is diagnosed with a genetic condition, you'll also have more time to prepare for the possibility of caring for a child who has special needs.
Other screening tests can be done later in pregnancy. An example is the quad screen, a blood test that's typically done between weeks 15 and 20. The quad screen can evaluate your risk of carrying a baby who has Down syndrome or Edwards syndrome, as well as neural tube defects, such as spina bifida. Some health care providers choose to combine the results of first trimester screening with the quad screen, although you won't learn the final results of this sequential testing until both tests are completed and analyzed.
Remember, first trimester screening is optional — and test results only indicate whether you have an increased risk of carrying a baby who has Down syndrome or Edwards syndrome, not whether your baby actually has one of these conditions. Before the screening, think about what the results will mean to you. Consider whether the screening will be worth any anxiety it may cause, or whether you'll handle your pregnancy differently depending on the results. You might also consider what level of risk would be enough for you to choose a more invasive follow-up test. If cost is a concern, ask your insurance provider for coverage details.
Risks
First trimester screening is a routine prenatal screening test. The test poses no risk of miscarriage or other pregnancy complications.
How you prepare
You don't need to do anything special to prepare for first trimester screening. You can eat and drink normally before both the blood test and the ultrasound exam.
What you can expect
First trimester screening includes a blood test and an ultrasound exam.
During the blood test, a member of your health care team simply takes a sample of blood by inserting a needle into a vein in your arm. The blood sample is sent to a lab for analysis. You can return to your usual activities immediately.
For the ultrasound exam, you'll lie on your back on an exam table. Your health care provider or a specially trained technician will place a slender, wand-like device in your vagina to send out sound waves and gather the reflections. The reflected sound waves will be digitally converted into images on a monitor. Your health care provider or technician will use these images to measure the size of the clear space in the tissue at the back of your baby's neck.
The ultrasound may take up to an hour. It doesn't hurt, and you can return to your usual activities immediately. Test results are typically available in less than a week.
Results
Your health care provider will use your age and the results of the blood test and ultrasound exam to gauge your risk of carrying a baby who has Down syndrome or Edwards syndrome. Other factors, such as your personal or family health history, also may affect your risk. Your health care provider or a genetic counselor will help you understand this risk and what it means for your pregnancy.
First trimester screening results are given as positive or negative and also as a probability, such as a 1 in 5,000 risk of carrying a baby who has Down syndrome. Generally, the test is considered positive if the risk is 1 in 300 or higher.
First trimester screening correctly identifies about 85 percent of women who are carrying a baby who has Down syndrome. About 5 percent of women have a false-positive result, meaning that the test result is positive but the baby doesn't actually have Down syndrome.
When you consider your test results, remember that first trimester screening only indicates your overall risk of carrying a baby who has Down syndrome or Edwards syndrome. A low-risk result doesn't guarantee that your baby won't have one of these conditions. Likewise, a high-risk result doesn't guarantee that your baby will be born with one of these conditions.
Often, abnormal test results are an invitation to consider further diagnostic testing — such as chorionic villus sampling or amniocentesis. These tests are more invasive than is first trimester screening and carry a slight risk of miscarriage, but they give more specific information about your baby's health. Having one of these tests is the only way to determine before birth whether your baby is affected by Down syndrome or Edwards syndrome.
The bottom line: Discuss the test results with your health care provider. A genetic counselor also can help you understand your options.
Last Updated: 2010-06-30
