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Genetic Screening, Ultrasound and Prenatal Diagnosis

Through genetic screening, parents can discover the likelihood of having a child with a genetic birth defect such as Down syndrome. Ultrasound is also used during pregnancy to screen for birth defects; however, neither procedure provides a formal diagnosis. If genetic screening or ultrasound indicates a baby could have a birth defect, parents may choose to have a prenatal diagnostic test performed to determine whether the baby in fact has a birth defect.

In addition to understanding genetic screening, ultrasound and prenatal diagnosis testing options, it may be helpful to learn more about the different types of birth defects, how they can be treated or managed and how they may impact children and their families. .

Your gynecologist will likely offer first trimester screening for Down syndrome. This screening is recommended by the American College of Obstetricians and Gynecologists (ACOG) for all pregnant women. With the exception of the first trimester screening, genetic testing is usually only offered to families who are considered high-risk for a particular genetic condition.

  • Carrier Screening: Even before you become pregnant, you and your partner can be screened to find out whether either of you carries a gene for certain inherited disorders. During the test, a blood sample or tissue sample will be swabbed from inside your cheek and tested. Carrier screening can be done before or during pregnancy.
  • First Trimester Screening: Between weeks 11 and 13 of pregnancy, you can undergo first trimester screening. This non-invasive evaluation includes a blood test and ultrasound of the fetus. The purpose of first trimester screening is to check for genetic birth defects related to your baby's heart or other genetic disorders such as Down syndrome.
  • Second Trimester Screening: Screens performed between 15 and 20 weeks are sometimes referred to as Triple or Quad screens. They include blood tests that measure two pregnancy-related hormones and ultrasound to check for genetic birth defects. Second trimester screenings are considered less accurate than first trimester screens.
  • Third Trimester Screening: The only screenings offered in the third trimester are routine checks conducted at every prenatal visit such as a urine test, blood pressure check, and measurement of the uterus and the fetal heartbeat. You will also be tested to determine whether you have a type of bacteria that can be passed to your baby during birth. If you test positive, you will take antibiotics during labor to prevent transmission of the bacteria to your baby.

Ultrasound Screening

Ultrasound is commonly used during pregnancy to monitor fetal development and determine a baby's due date. It is also used along with genetic screening to assess a developing baby's risk of Down syndrome and other birth defects including heart problems.

When ultrasound is used in this way, it is called a nuchal translucency scan or NT scan. It is performed when you are between 11 and 14 weeks pregnant because at this stage of development, your baby’s neck is still transparent. Therefore, the scan can measure the clear space in the tissue behind your baby’s neck. If the scan is abnormal, there will likely be more fluid accumulated at the back of the neck during the first trimester.

About the Procedure

During an NT scan, a small, handheld device that emits sound waves is rubbed across your abdomen by the clinician. These waves bounce off of the fetus and reflect back to the ultrasound machine, creating an on-screen image of the fetus and your uterus.

After the Procedure

After the NT scan, your gynecologist will discuss the results of the screening with you. It is important to know that sometimes, even babies with a normal ultrasound screen may end up having birth defects or genetic disorders that cannot be detected.

Prenatal Diagnostic Testing

If you have a positive result during any of these screenings, it does not confirm that your baby has an abnormality, but it does mean that you should discuss prenatal diagnostic testing with your gynecologist. This typically includes fetal testing such as chorionic villus sampling (CVS) or amniocentesis.