98: Prenatal Testing, Noninvasive Screening, Invasive Testing, and Carrier Screening

Key Points

• Summary:

  • Pregnancies found to be at greater than population (high) risk for a genetic disorder are offered invasive diagnostic testing by chorionic villus sampling (CVS) or amniocentesis. These diagnostic tests have a small risk of inducing a pregnancy loss but will diagnose a specific genetic disorder with greater than 99% accuracy.

  • Karyotype, FISH, or microarray are testing options for diagnosis of fetal genetic disorders on chorionic villus or amniotic fluid specimens.

  • Ancestry and family history are tools used to determine which pregnancies are at risk for specific genetic disorders and should help guide carrier screening.

  • The carrier state of some Mendelian disorders is frequent enough in the general population that offering screening to all pregnant couples and those presenting preconception is recommended.

  • Screening tests can be used during pregnancy, at as early as 10 weeks, to identify pregnancies at increased risk for common fetal trisomies such as those causing Down syndrome and Edwards syndrome (trisomy 21 and 18).

• Uses:

  • Invasive testing—CVS and amniocentesis

    • Provide a definitive diagnosis for pregnancies identified as having an increased risk of a genetic disorder.

    • Invasive testing is most commonly performed on women who are at an increased risk for a chromosome abnormality based on age-related risk, abnormal noninvasive screening results, and/or abnormal ultrasound findings.

    • Invasive testing is offered for disease-specific testing when the parent(s) are confirmed carrier(s), affected with a dominant genetic disorder, or known to carry a balanced translocation.

    • American College of Obstetrics and Gynecology, Guidelines, 2007

    • All women, regardless of age, should have the option of invasive testing.

    • Maternal age of 35 years alone should no longer be used as a cutoff to determine who is offered screening versus who is offered invasive testing.

General Descriptions

Types of Screening

Carrier Screening

Purpose: to identify carrier couples of Mendelian disorders

• Population screening for carriers of common Mendelian disorders

  • Some disorders occur frequently enough throughout the general population that carrier testing is offered to all pregnant couples, for example, cystic fibrosis (Table 98-1).

• Population screening based on ethnicity and race

  • The carrier state of certain genetic disorders occurs at an increased rate in specific ethnic or racial populations (Table 98-1). Since many of these disorders are inherited in an autosomal recessive fashion, identification of couples in which both parents are phenotypically normal but carry a mutated gene for the same disorder allows early decisions about future reproduction.

  • Preferably, carrier screening should be performed preconception or as early in pregnancy as possible. Turnaround time averages are 1 to 2 weeks.

  • Carrier screening is usually performed in a sequential manor—that is, if the mother tests positive, then the father of the pregnancy is tested. The exception to this is when time constraints, require a rapid analysis of both partners. If the parents are of different ethnicities, the father can be offered screening ...