Posts Tagged ‘chorionic villus sampling’

Screening vs. diagnostic testing—what’s the difference?

Friday, June 26th, 2015

DoctorPregnant_zps3ac96800If you are pregnant, you know that every visit to your prenatal care provider involves a number of tests. At each prenatal checkup, your provider checks your weight, blood and urine. But in addition to these routine tests, you will also be offered prenatal tests that can assess your risk to have a baby with certain birth defects.

The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women, regardless of age, be offered prenatal testing for Down syndrome and some other birth defects. There are two different types of tests that you can have: a screening test or a diagnostic test. It’s important to understand the difference between a screening test and a diagnostic test. Screening tests help evaluate the risk for certain birth defects, but they cannot diagnose a birth defect. Screening tests pose no risk to mother or baby. Diagnostic tests are highly accurate at diagnosing or ruling out specific birth defects. However, these tests may pose a very small risk of miscarriage.

Screening tests offered during pregnancy include:

  • Cell-free fetal DNA testing (also called noninvasive prenatal screening or testing): Some of your baby’s cells can be found in your blood. This test uses a sample of your blood to look at your baby’s DNA to check for certain genetic conditions. You can have this test after 10 weeks of pregnancy. This test is not recommended for women who aren’t likely to have a baby with a birth defect or who are pregnant with multiples.
  • First-trimester screening: Using a blood sample and ultrasound measurements, first-trimester screening can tell you if your baby is at risk for some birth defects, like Down syndrome. It is usually done between 11 to 13 weeks of pregnancy.
  • Maternal blood screening: This test measures four substances in your blood to determine the chance that a woman has a baby with certain birth defects like Down syndrome and neural tube defects. The test is done between 15 to 20 weeks of pregnancy.

Diagnostic tests are done by obtaining samples of your baby’s actual cells and therefore are more invasive. Diagnostic testing can detect most birth defects caused by a change in the number or shape of chromosomes. And testing for many inherited disorders can be done as well. However, not all birth defects can be detected.

Some women may choose to have diagnostic testing done instead of screening tests based on their age or family history. Other women may choose to start with a screening test and then, depending on the results, decide whether to have a diagnostic test. Prenatal diagnostic tests do carry a small risk of miscarriage (about 1 in 300-500).

  • Chorionic villus sampling (also called CVS): During this procedure, the doctor removes a small sample of tissue from the placenta. This can either be done transvaginally or transabdominally. You can get CVS at 10 to 12 weeks of pregnancy.
  • Amniocentesis (also called amnio): Your health care provider will use a needle to remove a small amount of amniotic fluid. Amnio is done between 15 to20 weeks of pregnancy.

Remember, all of these tests are optional. Make sure that you discuss your questions and concerns with your health care provider.

Questions? Send them to AskUs@marchofdimes.org.

 

Chromosomal abnormalities

Tuesday, August 20th, 2013

chromosomes1About 1 in 150 babies in the US is born with a chromosomal condition. Babies with chromosomal conditions have a problem in one or more of their chromosomes. Chromosomes are the structures that hold genes. Genes are part of your body’s cells that store instructions for the way your body grows and works. Genes are passed from parents to children.

Each person has 23 pairs of chromosomes, 46 in all. For each pair, you get one chromosome from your mother and one chromosome from your father.

Chromosomal conditions are caused by two kinds of changes in chromosomes:
1. Changes in the number of chromosomes—This means there are too many or too few chromosomes.
2. Changes in the structure of chromosomes—This means that part of a chromosome may be missing, repeated or rearranged.

Chromosomal conditions usually result from an error that occurs when an egg or sperm cell develops. It is not known why these errors occur and as far as we know, nothing that a parent does or doesn’t do before or during pregnancy can cause a chromosomal abnormality in their child. In some cases these kinds of changes can be inherited or passed from parent to child.

In most cases, an embryo with the wrong number of chromosomes does not survive. In such cases, the pregnant woman has a miscarriage. This often happens very early in pregnancy, before a woman may realize she’s pregnant. More than 50 percent of first-trimester miscarriages are caused by chromosomal abnormalities in the embryo.

The American College of Obstetricians and Gynecologists recommends that all pregnant women be offered a screening test for Down syndrome and certain other chromosomal abnormalities. Screening may consist of a maternal blood test done in the first trimester (at 11 to 13 weeks of pregnancy) along with a special ultrasound of the back of the baby’s neck. It also can be a maternal blood test done in the second trimester (at 15 to 20 weeks). A screening test helps identify pregnancies that are at higher-than-average risk of these conditions.

However, a screening test cannot diagnose Down syndrome or other chromosomal abnormalities. If a screening test shows that your baby may have a problem, your provider gives you a diagnostic test that can tell you if your baby actually does or does not have a chromosomal condition.

Diagnostic tests include amniocentesis or chorionic villus sampling. Your provider also can check your baby’s blood for chromosomal conditions after he’s born.

Each child born with a chromosomal condition is different. Some children with chromosomal conditions have intellectual disabilities or birth defects, or both. But some children with these conditions don’t have any serious problems. The problems depend on which chromosomes are affected and how they are affected.

If you or someone in your family has a chromosomal condition, or if you have a baby with a chromosomal condition, talk to a genetic counselor. A genetic counselor can help you understand the causes of chromosomal conditions, what kind of testing is available, and your chances of having a baby with these conditions.

New prenatal blood test

Thursday, December 13th, 2012

New prenatal tests can give some parents piece of mind that the baby they’re expecting doesn’t have a disorder.  The tests are given sooner and with greater accuracy and less risk of miscarriage than current tests.

The new tests, which use fetal DNA suspended in the mother’s blood, can test for chromosomal disorders such as Down syndrome (trisomy 21) and trisomy 18, and Rh blood incompatibility. They also can determine the baby’s gender, which allows health care providers to screen for disorders such as hemophilia.

During pregnancy, fetal cells and cell-free fetal genetic material circulate in the mother’s bloodstream.  This DNA can be used to screen for certain genetic disorders without having to perform invasive procedures such as amniocentesis or CVS (chorionic villus sampling).

At present four widely used procedures assist health care providers in prenatal diagnosis. They are:
Amniocentesis: a procedure that collects some of the amniotic fluid that surrounds the fetus for analysis. This is a diagnostic test.
Chorionic villus sampling (CVS): a procedure that obtains tissue from around the placenta. This is a diagnostic test.
• Maternal blood tests: screening tests that use mother’s blood to help identify problems with the fetal brain, spinal cord, intestines or chromosomes. These tests include alpha-fetoprotein (AFP), hCG and other hormones. As with all forms of screening, abnormal results require follow-up testing to make a diagnosis.
Ultrasound: a scan using sound waves to visualize the fetus.

Cell-free DNA methods of screening are a new option. “Prenatal diagnosis gives parents important information about the health of their baby and the status of their pregnancy. More often than not, prenatal screening reassures parents that their baby is healthy and that these disorders are not present,” says Joe Leigh Simpson, MD, March of Dimes senior vice president for Research and Global Programs.  “For a high-risk infant, prenatal diagnosis gives parents and health care providers options that may include planning for a health problem or arranging for delivery in a medically appropriate setting. The first step toward treating these problems is diagnosing them, and cell-free DNA methods, without the need for invasive measures, are welcomed.”

Dr. Lee P. Shulman of Northwestern University says, “while this represents a profound improvement in our ability to identify women at increased risk for carrying fetuses with the most common chromosome abnormalities, still, this technology represents a limited screening protocol and should not serve as a substitute for diagnostic tests such as chorionic villus sampling and amniocentesis, which provide the most accurate and comprehensive assessment by the direct analysis of fetal tissue.”

The American College of Obstetricians and Gynecologists and the American College of Medical Genetics recommend that information on prenatal genetic screening and diagnosis should be made available to all pregnant women and those considering pregnancy.

With all the above options for testing available, it’s important to discuss what is best for you and your pregnancy with your health care provider.

Blood test vs. amniocentesis

Thursday, January 13th, 2011

blood-testsResearchers are working on a new blood test that has the potential to diagnose Down syndrome in women who are at high risk for having a baby with this condition. If proven successful, this test might eliminate the need for the more invasive and risk-related tests of amniocentesis and chorionic villus sampling (CVS).  Both amniocentesis and CVS carry a risk of miscarriage.

A study conducted at the Chinese University of Hong Kong and recently published in the British Medical Journal (BMJ), included 753 pregnant women. Of these, 86 women were carrying a fetus with Down syndrome. There were no false-negative results with the new test. All babies with Down syndrome were detected. The study authors state that “If referrals for amniocentesis or chorionic villus sampling were based on the sequencing test results, about 98% of the invasive diagnostic procedures could be avoided. “

Further evaluation of this technology needs to be conducted before it is offered to the public, but this is exciting and hopeful science.  While the test won’t be available for at least another year, and the cost may be high, and currently it tests only for Down syndrome and not other conditions, it holds promise for the future.  It will be a new and safe screening tool to help us gather information about our developing babies.