Prenatal Testing for Birth Defects
A screening test, a diagnostic procedure, or no testing?
Helping you decide
Most pregnancies end with the birth of a healthy baby. However, every pregnant woman has a small chance of having a baby with a birth defect. Some of these birth defects can be found before birth by special tests known as "prenatal tests". It is not possible to find all birth defects before birth.
Prenatal tests for birth defects can be separated into screening tests and diagnostic procedures. Prenatal tests are used to learn about the health of your developing baby. These tests help to find specific kinds of birth defects, including chromosome abnormalities (like Down syndrome), and spina bifida. (Please visit our Genetics 101 section to learn more about chromosome abnormalities).
Screening tests and diagnostic procedures are available to all pregnant women, regardless of their age. The tests are optional, so the decision whether to have any prenatal testing is yours. Some of these tests are offered very early in pregnancy, so it is helpful if you make your decision as early in your pregnancy as possible.
This guide is intended to:
- Help you understand the differences between screening tests and diagnostic procedures.
- Provide you with the information you need to make the best decision for yourself.
What are prenatal screening tests?
Prenatal screening tests are tests that can help find out whether your baby has a higher or lower chance of being affected by a certain disorder.
Examples of screening tests: Please click on the links to learn more about the advantages and disadvantages of each test.
In pregnancy, the results of a screening test tell you the chance (probability) that your baby may have a certain condition. For example, the results of XAFP or FTS might show that there is a “1 out of 100” chance for a chromosome abnormality such as Down syndrome.
Screening tests like XAFP and FTS can help patients decide if they want a diagnostic procedure, such as those described below. Screening tests pose no additional risk for miscarriage to your pregnancy.
What are prenatal diagnostic procedures?
A prenatal diagnostic procedure can tell you with certainty if specific types of conditions are present in your baby.
Examples of prenatal diagnosis procedures: Please click on the link to learn more about the advantages and disadvantages of each procedure.
The results of prenatal diagnostic procedures like amniocentesis and CVS can tell you whether your baby has a chromosome abnormality such as Down syndrome. Although a woman of any age can have a baby with a chromosome abnormality, the chance is greater as she gets older. (See Table)
Diagnostic procedures are accurate, and can detect more than 99% of all babies with a chromosome abnormality. Prenatal diagnostic procedures have a risk of miscarriage (less than 1 in 100).
What are my choices for testing my baby for birth defects
during my pregnancy ?
You have a number of options for prenatal screening and diagnosis during your pregnancy. Some options are offered early in pregnancy. These options are summarized below.
Option #1
You may choose to have No Testing (and no diagnostic procedures) for birth defects during your pregnancy.
Option #2
You may choose to have First Trimester Screening (FTS) to screen your pregnancy for Down syndrome, trisomy 18 and trisomy 13.
Option #3
You may choose to have Expanded AFP blood screening (XAFP) to screen your pregnancy for Down syndrome, trisomy 18, neural tube defects, abdominal wall defects and Smith-Lemli Opitz syndrome (SLOS).
Option #4
You may choose to go directly to a Prenatal Diagnostic procedure such as amniocentesis or CVS to diagnose whether your pregnancy is affected with a chromosome abnormality.
Frequently Asked Questions
1) How can I decide between a screening test, a diagnostic procedure, or no testing for birth defects during my pregnancy?
Deciding whether to have a screening test, a diagnostic procedure or no testing for birth defects during your pregnancy is very personal. You may want to talk with your partner, doctor, nurse practitioner, nurse midwife, or a genetic counselor about the advantages and disadvantages of each type of test.
Some things to ask yourself:
Would knowing that your baby has a chromosome abnormality (such as Down syndrome) or certain other birth defects affect your decision to carry your pregnancy to term or help you feel more prepared for the birth of a baby with special needs?
- If NO, you may want no testing or diagnostic procedures for birth defects…
If you feel that there is no information about the health of your baby that you could be told while you are pregnant that would change your plans to continue your pregnancy, or make you feel more prepared for the birth of your baby, you may prefer to have no testing and no diagnostic procedures for birth defects during your pregnancy.
- If YES, read further below...
Some pregnant women feel that having a prenatal screening test will help them to decide if they want more testing, such as a diagnostic procedure. Most screening test results are reassuring, but in some cases, further tests may be recommended.
Do you want to know the chance that your baby could have certain birth defects (like Down syndrome, spina bifida)?
Are you willing to accept that even with a screening test, some babies with birth defects are not identified until after they are born?
- If YES, you may want a screening test…
Screening tests give you an estimate of the chance that your baby could have certain chromosome abnormalities and birth defects (without adding a risk for miscarriage), but do not give a “Yes or No” answer.
The vast majority (about 95%) of pregnant women will have screen negative test results. About 5% will have a screen positive result. Those women are offered a diagnostic procedure. The majority of women with screen positive results go on to have healthy babies.
Screening tests are not able to detect all babies with birth defects. For example, XAFP and FTS screening tests will find about 4 out of 5 pregnancies with Down syndrome, but about 1 out of 5 pregnancies with Down syndrome will not be picked up by these screening tests.
Do you want to know for certain whether your baby has a chromosome abnormality (like Down syndrome)?
Are you willing to accept a risk for miscarriage to learn this information?
If YES, you may want a diagnostic procedure…
Some women feel that even if the results of a screening test were reassuring, and showed that the chance of a chromosome abnormality was low, they would not feel comfortable enough. These women may feel that they need additional reassurance that their baby does not have a chromosome abnormality. These women may prefer to go directly to a diagnostic procedure without having a screening test first. Keep in mind that there is a risk for miscarriage after a prenatal diagnostic procedure.
2) Should I have all of these tests?
No, you should not have all of these tests.
First, you should decide whether you wish to have any screening tests or diagnostic procedures at all.
Then, if you decide to have a screening test, you should decide if you want first trimester screening (FTS) or second trimester screening (XAFP).
If you decide to have a diagnostic procedure, you should decide if you want CVS or amniocentesis.
If you choose one screening test (either FTS or XAFP) you do not have the other screening test.
For example, if you are certain you want XAFP screening, do not have FTS. If you choose to have FTS, do not have XAFP screening afterwards.
If you choose FTS, you will be offered a blood test known as “MSAFP-Only” at 16-18 weeks to test the pregnancy for other birth defects such as neural tube defects and abdominal wall defects.
If you choose a diagnostic procedure such as amniocentesis or CVS, you do not need FTS or XAFP.
4) Can I decline these tests?
Yes, you may choose to decline any or all of the prenatal screening tests and diagnostic procedures that you are offered.
5) What about prenatal ultrasound?
Prenatal ultrasound can provide important information about your pregnancy such as whether you are carrying twins, what your estimated due date is, and in certain cases, ultrasound can detect some physical birth defects. It is not possible to detect all chromosome abnormalities (like Down syndrome) with ultrasound alone. Ultrasound in prenatal diagnosis .
6) Who can I talk to for further information about prenatal testing for birth defects?
For more information about your options, you may talk to your doctor, nurse practitioner or nurse midwife, or you may ask to speak with a genetic counselor.
7) What do I do now that I have made my decision?
When you have made your choice about whether you want a screening test, a diagnostic procedure, or no testing for birth defects during your pregnancy, tell your doctor, nurse practitioner or midwife right away. Some tests are done very early in pregnancy.
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