Chapter 9: Methods of Prenatal Diagnosis

INTRODUCTION

Historically prenatal diagnosis was offered to all women who would be age 35 or greater at delivery. The exact reason that age 35 was chosen in the United States (in the United Kingdom age 37 was chosen) is not entirely clear, but it was not because there is a biological difference between women above, and those below, age 35 that causes a sudden increase in the risk of chromosome abnormalities in their offspring. Likewise, the age cut-off was not chosen to balance the risk of a chromosomal abnormality with the risk of procedure-related pregnancy loss, as the guidelines were established before the early studies were done to assess procedure related risk. The best explanations are those based on logistics: what resources were available to provide cytogenetic studies to the population of women. In the early 1970s when amniocentesis was introduced into practice the limited number of cytogenetic laboratories available in the United States could provide testing for about 5% of the pregnant population. At that time only 5% of the pregnant population was 35 or older at birth. Whatever the actual reason for defining age 35 or greater as “advanced maternal age,” it became the US standard of care until 2007 when the American College of Obstetricians and Gynecologists (ACOG) declared that age 35 should no longer be used as a criteria for whether a patient should be offered invasive prenatal testing.¹

Although the risk of chromosomal abnormalities such as Down syndrome (DS) does increase with maternal age, using maternal age as a screening criterion results in a very poor screening testing. With approximately 15% of the US population age 35 or older, there would be a 15% “false positive” rate to detect about 30% of DS fetuses. In 1975, when no other screening methods were available, a 5% false positive with a 30% detection was a more reasonable approach.

In counseling the patient described above, it is important to present the risk in terms that are understandable to the patient. Table 9-1 presents the risks of DS and all chromosomal abnormalities at both mid-trimester and at birth. At age 35 the mid-trimester risk (the standard time for most testing) of DS is 1/270, and for all chromosomal abnormalities 1/100. Giving these numbers as fractions or percentages (0.3% and 1%) is not truly understood by most patients. Putting these numbers in concrete terms, such as “1 in every 100 women of your age will have a chromosome abnormality in their pregnancy,” is more likely to convey the information in a way to allow an informed decision. The counseling session should include some discussion of the natural history of the disorders that are included in the category ...