Should I get a prenatal genetic screening test and what can I expect?

By: Cristela Hernandez, MD Maternal-Fetal Medicine

Pregnant women commonly ask their doctor, “Will my baby be healthy?” While the vast majority of babies are born healthy, all pregnancies are at risk for birth defects and genetic diseases. Two to three percent of babies are born with birth defects and 1 in 150 live births involve a chromosomal abnormality. Prenatal care involves screening for chromosomal abnormalities, genetic syndromes, and birth defects. Genetic screening is offered to all women during the first or second trimester of pregnancy. Subsequently, the doctor will review the mother’s results to determine, with as much accuracy as possible, whether a specific birth defect or genetic condition is present in her baby. Certain conditions can increase the mother’s risk, such as maternal age over 35, or a family history of birth defects and genetic disease. Keep in mind, that although many advances have been made, it is not possible to predict all birth defects or genetic disorders.

Prenatal genetic testing initially focused on the detection of Down Syndrome because it is the most common chromosomal abnormality, but testing has expanded to a broad range of other genetic disorders. Chromosomal abnormalities include extra or missing chromosomes such as Down Syndrome, which arises from an extra chromosome number 21. In addition, chromosomes can have abnormalities such as missing or duplicated fragments, or rearrangements from one chromosome to another. Other genetic disorders are caused by mutations in single genes, such as cystic fibrosis. Lastly, mutations in the mitochondrial DNA can also cause genetic disorders. The field of genetic disorders is complex and genetic counseling is very helpful when these abnormalities are encountered.

Prenatal genetic testing generally begins with a mother’s blood test and a fetal ultrasound exam. It is considered a screening test and does not entail any risk to the pregnancy, but is not considered a final diagnosis. Diagnostic invasive testing includes chorionic villus sampling and amniocentesis, and these can provide a definitive diagnosis. Chorionic villous sampling may be performed in the first trimester and an amniocentesis in the second trimester. These procedures can rarely lead to a miscarriage, so benefits and risks should be carefully considered.

Birth defects can be identified with fetal ultrasound exams and newer technology now includes 3- dimensional imaging. If a birth defect is suspected, a detailed fetal ultrasound is recommended. A referral to a Maternal-Fetal Medicine specialist can be helpful to address the multiple issues involved.

All of this information, in response to a concerned mother’s simple question, “Will my baby be healthy?” may seem overwhelming. It is important for every mother to discuss with her doctor the options in prenatal screening and her individual risk of having a genetic disease or birth defect. Prenatal genetic testing provides many benefits, including: reassuring parents when results are normal, identifying conditions which can be treated with fetal therapy before birth, and optimizing care for affected newborns. Such testing may prove very useful for the mother and doctors caring for her baby.