Chapter 8: Screening for Genetic Disorders in Pregnancy

INTRODUCTION

The phrase “screening for genetic disorders” generally refers to two forms of genetic testing. One approach is screening the patient and/or spouse to determine their carrier status for common genetic disorders, such as cystic fibrosis. Secondly, the genetic screening may refer to techniques that assess the likelihood the fetus is affected by a genetic condition or birth defect, such as Down syndrome (DS) or spina bifida. Although both are termed “screening,” the concepts behind each approach are significantly different.

CARRIER SCREENING

Screening for carrier status can either be directed to certain high-risk groups, or can be population based. Carrier screening for specific ethnic groups has been covered in Chapter 6 on Preconception Counseling. The approaches described previously for the patient contemplating a pregnancy are the same ones used for the pregnant patient. However, depending on the patient's gestational age, concurrent testing of the patient and her spouse may be necessary to provide adequate time for prenatal diagnosis, should both parents be found to be carriers of the genetic condition in question.

Population based screening, on the other hand, means that every pregnant woman, regardless of her ethnic background, is offered carrier screening. At the time of the writing of this book, only cystic fibrosis falls in this category. Although more common in individuals with northern European heritage, the mutant gene is found in all populations. This factor and the pluralistic nature of US society has lead the American College of Obstetricians and Gynecologists (ACOG) to suggest that all women should be offered carrier screening for cystic fibrosis.¹ One drawback to the current recommendation is that the recommended panel of mutations that should be tested for is based on the most common mutations in the “Caucasian” population, and may, therefore, detect very few carriers in an Asian populations.

In all forms of genetic screening, but especially in carrier screening, both the patient and the physician need to be well versed in the benefits and limitations of the testing. Using our Caucasian patient example, we know that the incidence of CF carriers in this population is 1 in 29. There are nearly 2000 mutations that have been found to cause cystic fibrosis, but testing for the 23 common mutations will detect approximately 90% of the carriers. If the patient is negative for the common CF mutations, her chances of having a child with CF are quite low, but it is not zero. Should she be found to carry a CF mutation, and her spouse have negative testing, their risk of having an affected child is approximately one in a thousand. Although a low risk, it is significantly higher than the population ...