Prenatal diagnosis The Genetics Center, Inc.

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Genetics & Prenatal Diagnosis:

Straightforward answers to
important questions

Down syndrome is the most common chromosome abnormality in newborns, with an incidence of approximately 1 in 800 births. The chance for having a baby with Down syndrome increases with the age of the mother, but chromosome abnormalities can occur at any age. A variety of screening and prenatal diagnostic options are available to address this important issue early in pregnancy.

Who should be concerned about chromosome abnormalities?

Every pregnant woman. Most babies with chromosome abnormalities have serious birth defects and significant developmental disabilities. Amniocentesis used to be offered mainly to women age 35 or older, but 80% of babies with Down syndrome are born to women under age 35.

How does screening work?

First trimester screening (at 11-13 weeks gestation) consists of a blood test and a specialized ultrasound examination of the fetus to measure the nuchal translucency. In the second trimester (14-22 weeks gestation) only a blood sample is necessary; typically three or four chemicals (“markers”) in the blood are analyzed, hence this is often referred to as triple or quadruple marker screening. Second trimester screening is also known as the alpha-fetoprotein (AFP) test, since AFP is one of the markers analyzed.

Some doctors recommend both first and second trimester screening. When the two screening tests are considered together, this is called sequential or integrated screening. On average, 95% of women will be “screen negative” and have low risks for carrying babies with either Down syndrome or trisomy 18, another serious chromosome abnormality. When a woman is carrying a baby with Down syndrome, the chance of being “screen positive” is greater than 85%. By contrast, ultrasound examination in the second trimester fails to detect any signs of Down syndrome 50% of the time.

A newly developed method of screening which promises to have higher sensitivity and specificity than the screening described above is called non invasive prenatal testing (NIPT) or screening using cell-free fetal DNA. As the name implies, analysis is performed on a blood sample by determining if the fetal DNA in the mother's blood appears to have excess chromosome 21 material. At present, NIPT is only advisable for women at high risk for Down syndrome for any of several reasons, including prior abnormal screening, maternal age, a previous child with Down syndrome, or fetal anomalies detected by ultrasound.

What if the screening result is normal?

A negative screening result is reassuring, particularly for women who might have considered amniocentesis. The laboratory typically provides a numeric risk and compares that to the risk based on maternal age. A negative screening result reduces, but does not eliminate the chance of carrying a baby with Down syndrome, and further screening and/or diagnostic testing can still be considered.

What if the screening result is abnormal?

Further testing, either additional screening or definitive testing should be considered. However, it is important to realize that most women who screen positive are carrying normal, healthy babies. They simply have a higher chance of carrying a baby with a chromosome abnormality than women who have a negative result.

Definitive prenatal diagnosis, either by amniocentesis (at 15-20 weeks gestation) or chorionic villus sampling (at 10-12 weeks gestation), is highly accurate. Each procedure has a very small risk (primarily of miscarriage) to the pregnancy.

Genetic counseling is available to help women make an informed decision about prenatal diagnosis. In addition to testing for chromosome abnormalities, prenatal diagnosis is available for many genetic and metabolic disorders, including, but not limited to, cystic fibrosis, Tay-Sachs disease, hemophilia, Duchenne muscular dystrophy, and sickle cell anemia.

What is the role of a geneticist?

Geneticists are medical specialists who help couples deal with genetic issues. The team of a clinical geneticist (who is a physician) and a genetic counselor (who has a graduate degree in genetic counseling) diagnoses genetic diseases, assesses risks for couples to have a child with a genetic disorder based on personal and family history, explains options for addressing abnormal results of first and/or second trimester screening, and arranges genetic and prenatal diagnostic testing. A comprehensive explanation of genetic issues (genetic counseling) is one of the priorities for the genetics team and is an integral part of any genetic consultation.