Fetal Genetic Testing

fetal testingAs more and more of the human genome is understood, genetic testing is becoming increasingly useful for diagnosing certain genetic disorders. There are good genetic tests for some congenital disorders, but not all disorders.

During a normal pregnancy, there are two ways to sample the fetus’ DNA that are in general use: amniocentesis and chorionic villus sampling. Finding enough fetal DNA in the mother’s blood for testing is still in the research stage. Embryos from in vitro fertilization are screened in a different way, and the science of this screening is also evolving.

Amniocentesis

The test is very safe when done during the second trimester, usually between 15 and 20 weeks of pregnancy. Amniocentesis is not as safe earlier in pregnancy, when there is less fluid and a greater chance the needle could injure the baby.

New genetic abnormalities are more likely as women get older, with 35 years of age the usual cut-off for testing. 

However, this means that a mother who finds out her baby will have a congenital disorder would have to decide if she wanted to terminate her pregnancy during the second trimester. This is more difficult emotionally and physically than an early pregnancy termination. When amniocentesis is done by a well-trained physician, the risk of miscarriage is 0.1% to 0.2%. Amniocentesis has been in use the longest. The procedure involves using a needle that is placed through the abdomen and into the uterus, to obtain amniotic fluid, which contains fetal cells. These will then be tested to see if the chromosomes are normal. Examining the chromosomes is called karyotyping.

Chorionic Villus Sampling 

Chorionic villus sampling (CVS) can be done earlier in pregnancy. It means getting a sample of placental membranes that have some of the baby’s cells. Chorionic villus sampling is usually done between 10 and 12 weeks. Ultrasound is used to determine whether the sample should be obtained through the abdomen or via the vagina and cervix. Again, with a skilled physician doing the procedure, the rate of miscarriage is approximately 0.2%.

While CVS can be done earlier, allowing for an easier termination of pregnancy if that is what is desired, more doctors are expert in amniocentesis.

Research is being done to find a way to isolate enough of the fetus’ blood in the mother’s circulation to be analyzed. Though, this kind of testing is not done in regular practice.

When a couple is undergoing IVF, their embryos will usually undergo some genetic screening before transfer. The success of the screening depends on the methods and experience of the center at which the IVF is performed. While some IVF centers report good results, others have found that taking a genetic sample of a tiny embryo may reduce the chance of a subsequent pregnancy. If a pregnancy occurs, further testing can be done later.

Why do people want genetic testing?
In some cases, there are inherited conditions in the family that they do not want to pass on to their children. Many of these conditions are more common in specific ethnic groups.

There are two of each chromosome. A dominant genetic condition will occur if a child gets the abnormal gene on one chromosome. A recessive condition will only cause illness if both copies of the chromosome have the abnormal gene. X-linked (recessive) disorders usually occur only in men who have one X chromosome and one Y chromosome. The X chromosome comes from the mother, so women carry X-linked diseases but do not have them.

Parents-to-be with family histories of any of these disorders can be checked for carrier status. They can use the information to decide if they want to do genetic testing on the fetus.

Some of the more common and best know conditions include:

  • Hemophilia (X-linked)
  • Duchenne muscular dystrophy (X-linked)
  • Huntington’s disease (dominant gene)
  • Cystic fibrosis (recessive gene, need two copies)

Some that are more common in certain ethnic groups include:

  • Sickle cell disease (more common in African-Americans and Hispanics)
  • Tay-Sachs disease (one of the disorders more common in Jewish people of Eastern European origin. Cajuns also have a higher risk of Tay-Sachs)
  • β-Thalassemia (in people of Mediterranean descent)
  • α-Thalassemia (more common in many Asian groups)

Others choose to be tested because of the possibility that a new chromosomal problem could have occurred with this fetus, or that they have a genetic abnormality that they do not know about. New genetic abnormalities are more likely as women get older, with 35 years of age the usual cut-off for testing. After 35 years, the rate of Down’s syndrome goes up sharply and continues to climb. Down’s syndrome and other somewhat similar problems are caused by an extra chromosome. There should be only two copies of each chromosome.

New genetic problems in the fetus that can be diagnosed include (but are not limited to):

  • Down’s Syndrome
  • Turner Syndrome
  • Klinefelter’s syndrome

Those with known familial disorders and older women are more likely to have abnormalities found.

Testing of fetal chromosomes will allow diagnosis of many familial as well as new genetic abnormalities. Of course, genetic testing will also reveal the baby’s sex.

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The American College of Obstetricians and Gynecologists recommends that all pregnant women be offered genetic testing. Those with known familial disorders and older women are more likely to have abnormalities found. There are also blood tests for the mother, as well as ultrasound examinations of the fetus that can indicate a problem. Chromosomal testing is suggested when any of the screening tests suggests a fetal abnormality.

What would prospective parents do if told the fetus had a genetic abnormality?

There are a limited number of problems that can be treated during pregnancy or immediately at birth with advance planning. In these situations, the knowledge can be used to help the baby. However, many of these congenital disorders cannot be cured. Some are fatal; others cause serious problems and the child will never be normal. There are some that can be managed, like sickle cell disease. There is progress in treatment of some congenital disorders, so there can be more hope now for a better life for an affected child.

Some parents-to-be want this information in order to decide whether or not to terminate the pregnancy. Others want to know, but will not terminate the pregnancy no matter what. There are some people who do not want this information at all. That is why each pregnant woman and her partner need to decide whether or not genetic testing is appropriate for them or not. In addition to getting the advice of a physician, expectant couples can also consult with counselors who specialize in this area.

 

What do you think?

Fetal Genetic Testing

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14 comments

  1. Profile photo of Ashleigh Ashleigh says:

    We haven’t done this with my past pregnancies and don’t plan on doing it with this one. To us it doesn’t matter. We will love our child no matter what.

  2. Profile photo of Eva Eva says:

    I never chose to have genetic tests done simply because it wasn’t important.

  3. Profile photo of Nerissa Nerissa says:

    I had the blood work done to test for chromosome disorders, it came back positive. I’m currently 16 weeks and 5 days pregnant. I’ve had an appointment set up with a high risk doctor because I have an underactive thyroid, PCOS, endometriosis and fibroids. Because the test came back positive, we’ll be seeing the high risk doctor on May 9th and getting a level 2 ultrasound done that will show the baby in more detail and they’ll be able to see if he has downs syndrome or not. plus they will be taking blood to separate my blood from the baby’s blood and test its DNA for the disorders. hopefully everything comes out normal.

  4. Profile photo of Alanna Alanna says:

    I’ve been able to get a lot of answers without an amnio. I’ve just had blood taken. I don’t want a needle in my amniotic sac!

    • me 2! whatever is wrong with or not wrong with my kid in the end is what i will deal with when the time comes. i’d rather not risk losing my baby just to find out if it has down syndrome

  5. Profile photo of Namaste Namaste says:

    We had no problems

  6. Profile photo of Jamila says:

    I never really got these tests and am happy about that. But I never had complications with my pregnancies

  7. Profile photo of Lindsay S. Lindsay S. says:

    I’m 35 besides being a type 1 diabetic, so my husband and I are opting for the testing due to the higher risk for complications and possible genetic problems. My doctor is also very proactive at explaining the tests involved and what each is used to determine, so I’m just glad these options are available.

  8. Profile photo of Dottie Dottie says:

    My original doctor never offered the testing to me which made me very angry because I plainly stated that I wanted it due to a long family history of genetic problems. By the time I changed to my current doctor it was too late to do most of the test. Thankfully my new doctor is more involved and has made me feel better about the chances of my little one having one of the illnesses. The ultrasound looked great and the only problem is the baby is just big!

  9. Profile photo of KITTYNOLAND KITTYNOLAND says:

    I’m bummed that they did not mention MaternaT21. This is bloodwork which is done on the expectant mother. It looks for particular proteins and gives statistics for the main three chromosomal abnormalities, as well as the sex of the child. It is done at 12 weeks and often in conjunction with a NT (nuchal translucency). I’m 37, second pregnancy, and this is the limit of testing that we will do. With my first pregnancy, I had myself checked to see if I carried recessive or dominant genes for Cystic Fibrosis, Fragile X, and SMA. GOOD LUCK!

  10. I’m over 40 and we would like to know but we have already discussed if we have a special needs child we are happy and prepared to take on those challenges. Our concern is to be prepared if there is a chromosomal abnormality that would not be compatible with life and for that we would want to be prepared. We pray all is well our OB physician has assured us I’m not the 1st to be pregnant at my age and still have a normal pregnancy.

  11. Profile photo of AspenXIzzard AspenXIzzard says:

    I agree I would rather not know some things but I do see why others would be important.

  12. Profile photo of Anya says:

    Sometimes it’s best not to know things that can’t be prevented I didn’t get Down’s syndrome ultrasound done because I was keeping the littler man no matter what. It can’t be prevented so why stress out for the whole pregnancy about it and cause more harm to the baby and your health.

  13. Profile photo of samantha samantha says:

    My doctor did bloodwork for our genetic testing last wednesday. Im anxiously waiting to hear the results. Praying for a healthy baby ♡ will she be able to tell the gender from the bloodwork? Baby kepts its little legs crossed at our last ultrasound

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