Women’s Health Care Resources

Obstetrics Services

Prenatal Genetic Testing

During your pregnancy, we wish to reassure you as much as possible that your baby is free from chromosomal and developmental problems. While no screening is perfect, you will find the available screening tests below.

There are several testing options that some patients elect to do while others do not. Please discuss these with your provider to make the best decision for you and your family.

 First Trimester Screening

Our perinatology service offers first trimester screening. This non-invasive screening is a combination of an ultrasound of the baby and a blood test on the mother. The ultrasound examines the thickness of skin along the baby’s head, neck, and back called the “nuchal translucency”. If it measures within the normal range, aneuploidy such as Trisomy 21 (Down Syndrome), Trisomy 18 (Edwards Syndrome) or Trisomy 13 (Patau Syndrome) is much less likely. If it measures thicker than expected, there is a greater association with a chromosomal abnormality. A normal nuchal translucency measurement excludes up to 70% of Down’s infants. You can elect to do this ultrasound alone or in combination with the blood tests listed below for an earlier screening for birth defects.

The other portion of the full first trimester screen involves a blood test for HCG and pregnancy-associated plasma protein A (PAPP-A). Each first trimester factor (nuchal translucency, blood studies, and statistical tables) provides valuable information and together, the results give us a more complete picture of the risk of aneuploidy. If normal, this combination of tests excludes up to 85% of Down’s infants.

Fetal DNA Prenatal Testing

This is a non-invasive test that screens for specific chromosomal abnormalities that can impact the health of your baby. This can be performed any time after 10 weeks and requires a blood draw from the mother to measure the relative amount of fetal chromosomal material in the maternal blood. It is usually performed in combination with the “nuchal translucency” testing (above).

It is a non-invasive test with no increased risk for miscarriage and screens for the most common, medically impactful conditions (including Trisomy 21, 18, 13). It has been shown to have detection rates up to 99% and false positive rates as low as 0.1% for trisomy 21, 18 and 13. A negative result indicates that your pregnancy is not at an increased risk for the conditions screened. If the results are positive, your pregnancy may be at an increased risk for a chromosomal abnormality and we will promptly refer you to a maternal-fetal medicine specialist as well as a genetic counselor for further evaluation.

Carrier Testing

Carrier testing is a blood test that analyzes your DNA to provide specific information about your child’s risk for certain genetic disorders. This blood test screens for common inherited conditions (Cystic Fibrosis, Spinal Muscular Atrophy, Fragile X). If you are a carrier, that means you have a change in one copy of a gene that increases the risk of your child inheriting the associated disorder.

There are several options for carrier screening with multiple panels for different conditions. Talk with your provider if you are interested in carrier screening.

Quad Screening/Alpha-Fetoprotein (AFP)

This common blood test screens for neural tube defects of the spine or skull such as spina bifida. This test can be done between 15 and 20 weeks gestation. It uses multiple factors such as maternal age, maternal weight, gestational age, and four blood tests. If the value is high, neural tube defects (such as spina bifida and hydrocephaly) are suspected. A low value is somewhat suspicious for aneuploidy. If either high or low values are seen, a full ultrasound is performed as soon as possible for reassurance.

Invasive Testing

Chorionic Villus Sampling (CVS) is the earliest direct testing and may be performed around 13 weeks gestation. This consists of a needle sampling of cells drawn from the developing placenta. Within 24 hours, the most common forms of aneuploidy (Trisomy 13, 18, 21) can be assessed along with fetal sex. After a week or more of cell cultures, a broad range of chromosomal patterns associated with fetal abnormalities can be assessed.

Amniocentesis may be performed between 15 and 17 weeks. This procedure involves needle penetration into the amniotic sac and gathering cells that are dispersed in the amniotic fluid. The chromosomes are analyzed in the same fashion as the chorionic villus biopsy.

These procedures are performed by maternal-fetal medicine specialists. Even with the greatest care and skill, procedure-related pregnancy loss occurs at a rate of 0.1 to 0.3% when performed by experienced operators.

20 Week Fetal Anatomic Survey

We perform a comprehensive ultrasound at 20 weeks gestation. During this ultrasound, we assess for normal fetal anatomy as well as growth, fluid levels, placenta location, cervical length and fetal sex (if you want to know!) Among other rare anomalies, characteristics of aneuploidy might be noted during this examination. Major anomalies in your baby are not frequent occurrences, but the 20 week survey is effective in detecting many of them.

First trimester screening and the 20-week fetal survey are not the final diagnostic modalities to detect Trisomy 13, 18 or 21. At least 30% of fetuses with Down’s and up to 10% of fetuses with Trisomy 13 and 18 will not have any identifiable abnormalities on ultrasound.

Final Considerations

How complete do you want your screening? This depends on your desire to know of any possible complications to prepare for the birth of your child. You may feel you do not need this information in advance or that the testing process may be stressful. You may decide you would like to know if there are any problems so you can line up special care or handle insurance issues beforehand.

There is no right or wrong decision, only what you and your partner determine is right for you. Please feel free to discuss any questions you have with your OB provider.

Prenatal Genetic Testing Chart


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