What are my options for genetic testing?

Why does it matter?

Update 2016

Fetal cell DNA Harmony Prenatal Test now available

Having a healthy baby is a parent’s greatest hope. While most babies are born healthy, some babies are born with health problems. Chromosomal irregularities and neural tube defects are some of the conditions that can detected early in pregnancy. There are several screening tests available to determine the likelihood of chromosomal problems and neural tube defects, and two diagnostic tests to confirm or rule out chromosomal anomalies.

Genetic screening helps parents and providers determine whether more invasive testing such as amniocentesis is indicated. A family might choose to proceed to diagnostic testing in order to obtain diagnostic information to prepare emotionally and financially for a high needs child, or to opt to terminate the pregnancy. For those who continue their pregnancy, an early diagnosis can also allow time for parents to become educated about the condition and to choose the best delivery and treatment options for their baby. While genetic screening is common in pregnancy care, families have the option to decline testing. Genetic testing can be stressful and may be waived when parents are certain that they would continue their pregnancy regardless of the results.

Common abnormalities

What is Trisomy 21 (Down syndrome)? Trisomy 21, or Down syndrome, is a disorder that is characterized by the presence of an extra 21st chromosome, causing varying degrees of mental retardation, and an increased risk of physical defects like heart problems. Women of any age can give birth to a baby with Trisomy 21, though the risk increases with age. This condition affects approximately 1 in 700 babies born.

What is Trisomy 18 (Edward syndrome)? Trisomy 18, or Edward syndrome, is a disorder that is characterized by the presence of an extra 18th chromosome. Many of these babies do not survive to delivery, or die soon after. A small number are able to survive but require significant care and are not able to live independently. This condition is rare, and affects approximately 1 in 5,000 babies born.

What is Trisomy 13 (Patau syndrome)? Trisomy 13, or Patau syndrome, is a disorder that is characterized by the presence of an extra 13th chromosome. These babies typically have serious heart problems as well as other structural problems. The severity of these problems varies depending how many cells have the extra chromosome. This condition is very rare, and affects approximately 1 in 16,000 babies born.

What is a neural tube defect (NTD)? This term includes spina bifida (open spine) and anencephaly. Spina bifida results from a failure of the spine to close completely over the spinal cord. The problems associated with spina bifida will vary with size, location, and nature of the opening of the spine. Anencephaly is a failure of the skull and brain to fully develop. It is incompatible with life. Screening tests usually detect the more severe forms of neural tube defects.

Testing options

Ultrasound

Ultrasound

Ultrasound uses sound waves to create an image of the baby. The routine fetal survey looks in detail at the baby’s anatomy and can identify a wide variety of structural abnormalities, including “soft signs” of trisomy 21 (Down syndrome).

Routine fetal ultrasound is a screening test.  It is usually performed first at 6-12 weeks for dating and viability purposes and then again at 16 to 22 weeks for an anatomical scan.  Ultrasound is useful for detecting certain abnormalities of development such as heart defects, problems with the placenta, and can also often detect Down Syndrome.

Combined screen

Screening test performed in the first trimester with a blood draw and an ultrasound generally done on the same day around 11-13 weeks. This information is used to predict the likelihood a pregnancy is affected by trisomy 21 (Down syndrome) or trisomy 18.

Receive results in the first trimester (or very early second trimester).

Performance Issues/ Risks:  It does not give information about neural tube defects (NTD) so the maternal alpha-fetal protein (AFP) test is available later in pregnancy to test for this.  It also has a higher false positive rate than the IPRP.  It has a 3-5% false positive rate (3-5% of normal fetuses will show up as having the condition).

Why this test?  It detects 85-91% of trisomy 21 (Down syndrome)  in the first trimester so you have more time to make decisions or pursue other testing.

IPRP

aka Integrated Prenatal Risk Profile

Screening test performed in the first and second trimesters: a blood draw and an ultrasound in the first trimester, and another blood draw in the second trimester. This information is used to predict the likelihood a pregnancy is affected by Trisomy 21 (Down syndrome), trisomy 18, or neural tube defects

Receive results in the second trimester.

Performance Issues/ Risks:  Because information from the second trimester is combined with information from the first trimester in order to give you the most accurate risk calculation, you have to wait until the second trimester to get results. The combined screen uses only first trimester information.

Why this test?  It detects 85% of trisomy 21 (Down syndrome) and 80% of neural tube defects, with only a 1% false positive rate (better than Combined Screen).

Maternal Serum aFP

Screening blood test between 16-24 weeks to assess the amount of alpha-fetal protein in the mother’s blood, which is used to predict the likelihood a pregnancy is affected by a neural tube defect. Often ordered in conjunction with the Combined Screen, which does not screen for neural tube defects.

Performance Issues/ Risks:  It does not give information about chromosomal defects

Why this test?  It detects 75% of neural tube defects and abdominal wall defects with a 3% false positive rate.
If you only received the Combined Screen in the first trimester, this test can be added on to test for NTD.

Quad Screen

Screening blood test performed between 15-20 weeks to identify women more at higher risk for having a baby affected by trisomy 21 (Down syndrome), trisomy 18, and neural tube defects.

Performance Issues / Risks:  Not useful for risk estimates with twins. Does not screen for all serious chromosomal abnormalities.

Why this test?
Detects 70% of trisomy 21 (Down syndrome)
Detects 80% of neural tube disorders
5% false positive rate, meaning 5% of normal pregnancies get a positive result

Cell-free Fetal DNA

Screening blood test performed between 10-22 weeks that counts the amount of chromosome 13, 18, 21 and Y material in the fetal DNA that is circulating in the mother’s blood. This test has been validated for people who fit in one or more of four categories:

  • those of “advanced maternal age” (generally 35+, younger if it’s a multiple pregnancy)
    those with a personal or family history of chromosomal abnormalities
    those who have had an ultrasound that suggests the baby has an abnormal number of chromosomes
    or those who have had a positive 1st or 2nd trimester screen (combined screen, integrated screen, quad screen)

Why this test?  This test can serve as an “in between” in order to avoid an amniocentesis, or as a more accurate screening test for those at higher risk.

Performance ratings:
Detects 99% of trisomy 21, with 0.2% false positive
Detects 99% of trisomy 18, with 0.4% false positive
Detects 91% of trisomy 13, with 0.3% false positive
Is 99% accurate at detecting that a fetus is male (has Y chromosomes)
Can also detect  some syndromes where there are too many or too few sex chromosomes, like Turner Syndrome (45,X), Klinefelter (47,XXY), Triple X (47,XXX), Jacob’s Syndrome (47,XYY)

Amniocentesis
Harmony Prenatal Test

Update 2016

Harmony identifies and separates fetal cells circulating in the maternal blood stream and then tests the DNA in the fetal cells.  It is more accurate than the other testing options because of this.  It can be drawn by your midwife from 10 weeks on.  It is also covered by all insurances and the test never costs more than $150 out of pocket.

Ask your midwife about your options.

www.harmonytest.com

Thanks to One Sky Family Medicine at 6300 9th Ave NE Suite 300, Seattle, for use of this document.

Genetic screening tests

versus diagnostic tests

Screening

Screening is not a diagnosis.  It is a probability of the presence of a certain condition based on good evidence and good science.

There is always the possibility of a “false positive” or a “false negative”.  A false positive means that the screening test shows the high likelihood (or presence) of a condition when, once further testing is done or the baby is born, that condition is not there.  A false negative means that the screening test shows the low likelihood (or absence) of a condition when, once further testing is done or the baby is born, that condition is actually there.

It is important to know that screening tests are changing and becoming more accurate as technology improves.

Diagnostic tests

A diagnostic genetic test is generally a more invasive and expensive test and they often carry more risk to the fetus.  This is why diagnostic tests are done after the screening tests.  A diagnosis of a fetal abnormality is made based on direct chromosomal material of the baby.

After the initial screening test (e.g. Quad screen or Fetal DNA) is positive for a condition, the next step is usually an amniocentesis to draw amniotic fluid out of the uterus and test the baby’s DNA.

It is important to know that the possibility of false positives and negatives still exist, but are much less likely than with screening tests.

Genetic counseling

We refer any client with an abnormal screening or diagnostic test, with a history of genetic problems, with specific concerns, or who requests it to a genetic counselor.

Any client can request genetic counseling.