Genetic screening is used to determine whether a couple is at increased risk of having a baby with a hereditary genetic disorder.
Genetic screening is not for everyone. Counseling is recommended when one or both partners know they have a genetic disorder or have family members who have or may have a genetic disorder. Genetic screening involves assessing the couple's family history and may involve determining whether a prospective parent who does not have symptoms of a particular disorder has a gene for that disorder (carrier screening).
Family History Assessment
To determine whether having a baby with a genetic disorder is likely, doctors ask the couple about disorders that family members have had and about the cause of death in family members. Information about three generations is usually needed. Doctors also ask about the health of all living first-degree relatives (parents, siblings, and children) and second-degree relatives (aunts, uncles, and grandparents). Information about miscarriages, stillborn babies, or babies who have died soon after birth is also helpful, as is information about intermarriages among relatives and ethnic background. If the family history is complicated, information about more distant relatives may be needed. Sometimes doctors review the medical records of relatives who may have had a genetic disorder.
Carrier screening involves testing people who do not have symptoms of a particular disorder but may have one nonsex (autosomal) recessive gene for that disorder and one normal gene (see Genetics: Transcription and Translation). (If a disorder results from an abnormal autosomal recessive gene, a person must have two of the abnormal genes to develop the disorder and have symptoms.) Prospective parents are screened if they have a family history of certain disorders or characteristics (such as ethnic background) that increase the risk of having certain disorders.
Screening is performed only if the following criteria are met:
- The disorder is very debilitating or lethal
- A reliable screening test is available
- One or both parents are likely to be a carrier because the disorder runs in the family or is common in their ethnic, racial, or geographic group
- The fetus can be treated, or reproductive options (such as abortion or elective sterilization) are available and acceptable to the parents.
Mostly sickle cell anemia, the thalassemias, Tay-Sachs disease and cystic fibrosis meet these criteria.
Screening usually consists of analyzing a blood sample. But sometimes a sample of cells from the inside of the cheek is analyzed. The person provides the sample by swishing a special fluid in the mouth, then spitting it into a specimen container. If both parents carry one abnormal autosomal recessive gene for the same disorder, their baby may be born with the disorder. In such cases, the chance of a baby receiving an abnormal recessive gene from each parent is 1 in 4 for each pregnancy.
If carrier screening indicates that both parents have an autosomal recessive gene for the same disorder, the parents may decide to have prenatal diagnostic testing. That is, the fetus may be tested for the disorder before birth. If the fetus has the disorder, treatment of the fetus may be possible, or termination of the pregnancy may be considered.