Three types of carrier screening may be ofered: ethnicity-based screening, panethnic screening (performed regardless of ethnicity), and expanded carrier screening-which is a type of panethnic screening performed for a larger number of conditions, potentially 100 or more. The goal of screening is to provide individuals with meaningful information to guide pregnancy planning according to their values (American College of Obstetricians and Gynecologists, 2017 a). Each type of screening has beneits, risks, and limitations. For example, so many disorders are included in expanded carrier screening panels that more than 50 percent of those screened may be identified to be carriers for at least 1 condition. his can cause anxiety for families and may pose challenges if genetic counseling resources are limited. Recognizing that each type of screening is an acceptable strategy, it is recommended that obstetrical providers develop a standard approach to ofer one of these three types of carrier screening to pregnant women and couples considering pregnancy (American College of 0bstetricians and Gynecologists, 2017 a). All carrier screening is optional and should be an informed choice.
Couples with a personal or family history of a heritable genetic disorder should be ofered genetic counseling. They are provided an estimated risk of having an afected newborn and given information concerning beneits and limitations of available prenatal testing options. Prenatal diagnosis may be available if the diseasecausing mutation or mutations are known. The publicly unded Genetic Testing Registry website contains detailed information regarding more than 10,000 genetic conditions and 48,000 genetic tests (www.ncbi.nlm.nih.gov/gtrl). hat said, many genetic disorders are characterized by a high degree of penetrance but variable expressivity. Thus, prediction of phenotype may not be possible, even when family members are afected. Common examples include neurofibromatosis, tuberous sclerosis, and Marfan syndrome. There are also conditions for which risk may be refined by detection of associated sonographic abnormalities or by gender determination if X-linked.
Ethniciy-based carrier screening is ofered for certain autosomal recessive disorders that are found in greater frequency in specific racial or ethnic groups (Table 14-10). The ounder eect occurs when an otherwise rare gene is found with greater frequency in a certain population and can be traced back to a single family member or small group of ancestors. his phenomenon may develop when generations of individuals procreate only within their own groups because of religious or ethnic prohibitions or geographical isolation. Because it is becoming increasingly diicult to assign a single ethnicity, a panethnic screening panel is another option.
The American College of Obstetricians and Gynecologists (2017 a) has developed the following criteria for expanded carrier screening panels: 1.
Conditions included in the panel should have a carrier frequency of at least 1: 1 00, which correlates with a population frequency, at minimum, of 1 :40,000.
2.
Conditions should have a well-defined phenotype, detrimental efect on quality of life, cognitive or physical impairment, early onset, or require surgical or medical intervention.
3.
Conditions primarily associated with disease of adult onset are not recommended for inclusion.
4.
If an individual is at increased risk for a speciic condition, such as Tay-Sachs disease or 3-thalassemia, the provider should consider that the test included in the panel may not be the most sensitive one for that condition.