Case Based Pediatrics For Medical Students and Residents
Department of Pediatrics, University of Hawaii John A. Burns School of Medicine
Chapter IV.1. Prenatal Genetic Screening and Testing
Greigh I. Hirata, MD
January 2002

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Mrs. H is a 37 year old G1P0 with irregular menstrual periods. She presents to you early in her pregnancy for prenatal counseling. Her significant family history includes a brother with unexplained mental retardation and a niece with beta-thalassemia major. She is of mixed Asian/Caucasian ethnicity. Her husband and the father of the baby is a 49 year old African-American with no significant family history. She seeks advice with regards to prenatal screening for birth defects and/or prenatal testing.


This case brings up important issues regarding prenatal screening and testing. It must be remembered that screening tests are designed to identify a high risk population from the general population. Screening tests are generally inexpensive and noninvasive. Prenatal testing is designed to answer a specific question in a population at high risk. Definitive diagnostic testing is typically expensive and invasive.

A complete prenatal evaluation begins with a careful medical and family history. When used in conjunction with a thorough physical examination (ultrasound evaluation) and laboratory testing, the practitioner is better able to individualize the patient's risks and recommend the appropriate diagnostic tests.

In this section, we will discuss the appropriate steps in risk assessment beginning with the family history. One typically begins the assessment by asking questions regarding other family members. A pedigree is constructed which includes three generations; grandparents, uncles, aunts, cousins, parents and siblings of the proband (the index case), in this case the fetus. Significant information includes histories of birth defects, genetic diseases, unexplained stillbirths, and unexplained mental retardation. It is important to recognize combinations of abnormalities and illness and patterns of inheritance that may require referral to a geneticist for diagnosis and further evaluation.

Also significant is the ethnic background of both parents. Specific genetic diseases are much more common in certain ethnic groups. For example, southeast Asians and Mediterraneans are at risk for thalassemia and glucose-6-phosphate dehydrogenase deficiency, African-Americans are at risk for beta thalassemia and sickle cell disease, Ashkenazi Jews are at risk for Tay-Sachs disease and have a genetic predisposition to certain types of cancers, and northern Europeans are at risk for cystic fibrosis. Screening tests for the carrier status are readily available for each of these disorders and should be performed prior to any prenatal diagnostic test if the couple is at risk.

The most obvious screening test for fetal aneuploidy is maternal age. This association has been well characterized and has lead to the recommendation that invasive genetic testing be offered to any women 35 years or older at the expected date of delivery. Although every pregnant women is at risk for aneuploidy, this age cutoff offers the most efficient and effective method for determining candidates for prenatal testing. Advanced maternal age is also a risk factor for increased maternal morbidity and mortality primarily related to increase rates of pregnancy complications such as preeclampsia and gestational diabetes. Pregnancy wastage, unexplained stillbirths and other adverse perinatal outcomes are also increased. As women increasingly delay childbearing to later years, clinicians should become aware of these risks to better counsel their patients.

Advanced paternal age (45 years or greater) places the fetus at risk for new autosomal dominant mutations. Examples of these genetic disorders include achondroplasia, Marfan syndrome and certain types of osteogenesis imperfecta. Unfortunately the exact occurrence risk is unknown and invasive prenatal testing are not available for many of these genetic disorders.

One of the greatest breakthroughs in prenatal diagnosis has been the emergence of maternal serum screening in the identification of pregnancies at risk for chromosomal aneuploidy and birth defects. Originally designed as a test for spina bifida and ventral abdominal wall defects, these tests are performed at 15-20 weeks gestation.

Abnormal levels of the maternal serum markers human chorionic gonadotropin (hCG), alpha-fetoprotein (aFP), and unconjugated estriol (uE3) are associated with trisomy 21 and 18. Moreover, these markers are predictive of aneuploidy independent of maternal age related risks. This has lead to the development of calculated individualized risk for these specific aneuploidies (utilizing maternal age and maternal serum biochemical markers). This test, also known as the triple screen, is rapidly replacing maternal age alone as an indicator for invasive genetic testing. It has enabled clinicians to identify women at risk for aneuploid fetuses who are less than advanced maternal age (< 35 years old). Conversely, many women greater than 35 years old have been reassured by risk reduction and thus have avoided placing the pregnancy at risk with invasive genetic testing. The maternal serum screening is also useful in identifying those pregnancies at risk for specific birth defects such as neural tube defects and ventral abdominal wall defects. In this case, the maternal serum marker aFP is elevated. Typically hCG and uE3 are unaffected.

Other etiologies for abnormal test results include fetal demise, multiple gestations, and incorrect gestational age determination. By identifying these problem pregnancies and evaluation by ultrasound, clinicians are better able to intervene or anticipate pregnancy complications. In the absence of recognizable explanations for an elevated maternal serum aFP level, there has been a noted increased risk for adverse perinatal outcomes such as preterm birth, intrauterine growth retardation, oligohydramnios and stillbirth. Unfortunately treatment protocols have been unsuccessful in significantly improving the outcomes of these high risk pregnancies.

The future of maternal screening involves earlier identification of pregnancies at risk either by serum screening or ultrasound as well as noninvasive methods for prenatal diagnosis. We will now explore tests which will become clinically available in the not too distant future. Researchers are busy investigating promising new maternal serum markers applicable earlier in pregnancy. Like hCG, aFP, and uE3, these markers are independent predictors of aneuploidy, primarily trisomy 21. The utilization of these markers is estimated to increase sensitivity rates by approximately 5%. In the near future, maternal age in conjunction with serum markers such as inhibin, pregnancy associated plasma protein-A (PAPP-A), and urinary human chorionic gonadotropin-core may be used alone or in combination as screening tools in pregnancy. Nuchal translucency (significant swelling of the nuchal area seen on ultrasound) occurs in approximately 70 % of aneuploid fetuses at 10-14 weeks gestation independent of maternal age risks. Evaluation of early gestations by sonography may identify these fetuses at risk. The promising future direction of early pregnancy screening will probably involve a combination of nuchal translucency, maternal age, and serum screening utilizing beta-hCG and PAPP-A in identifying these abnormal gestations.

Fetal cells normally appear in the maternal circulation. Because of breakthroughs in isolating these cells from the maternal circulation and genetic technology enabling testing minute samples of tissue, noninvasive prenatal diagnosis is a real future possibility. Prenatal diagnosis would therefore be possible without placing the fetus at risk (i.e., an amniocentesis may not be necessary in most instances). Clinical trials are currently underway investigating the feasibility of this new technology.

Ultrasonography has revolutionized the field of prenatal diagnosis since its introduction into clinical medicine in the 70's. The previously visually inaccessible uterus has been revealed by this noninvasive technology. It is important to realize that sonography can be used not only as a screening tool but also a diagnostic tool. The value of ultrasound as a screening tool is controversial most likely because it is highly dependent on the skill of the examiner.

Prenatal testing involves invasively obtaining samples from the fetus or fetal tissues. The cells can then be analyzed for a variety of tests including karyotype analysis, molecular DNA analysis, and chemistries and cultures. We will now explore the different prenatal testing procedures that are currently available.

Amniocentesis is generally performed at 15-20 weeks gestational age. This test involves sonographic localization of the placenta, fetus and amniotic fluid. Under ultrasound guidance, a spinal needle is inserted percutaneously into the amniotic sac withdrawing approximately 20 cc's of amniotic fluid. Within this fluid, fetal cells from the fetal skin, urinary system and amniotic membranes are spun down and collected. The cells are then grown in culture for approximately 5-6 days and arrested in the metaphase of the cell replication cycle. After fixation and staining, the chromosomes are identified and counted to assess the number and gross structure. Typically, humans have 22 pairs or autosomes and two sex chromosomes for a total of 46 chromosomes. As with any invasive tests, there is a risk for miscarriage of approximately 1:200-300 procedures performed.

Chorionic villus sampling can be accomplished in the first trimester by sampling the placenta either transcervically or transabdominally. Since the placenta is fetal in origin, karyotype analysis of the placental cells will most often accurately reflect the fetal chromosomes. This test is typically performed at 10 to 13 weeks gestation. The major advantage to this procedure is the earlier gestational age at the time of diagnosis. The draw back is a slightly increased risk for miscarriage of approximately 1:75-100 procedures performed.

Percutaneous umbilical blood sampling (PUBS) of fetal blood is sometimes required to evaluate fetal anemia, fetal infection, and rapid fetal karyotype. This procedure allows direct evaluation of fetal blood and serum. The procedure is performed much like that of an amniocentesis under ultrasound guidance. The needle is directed to the umbilical cord and blood removed directly from the fetal blood vessels. Because the target is much smaller, skill at imaging the vessel and directing the needle is an absolute requirement. Blood withdrawn can be analyzed much like any other blood sample. In addition, since the white blood cells in the fetal circulation are actively dividing, karyotype analysis is accomplished much quicker, often without requiring many days of cell growth. This procedure can be performed as early as 16 weeks gestational age. The miscarriage risk for this procedure is approximately 1%.


Questions

1. Pertinent family history includes all of the following except:
. . . . a. Ethnic background
. . . . b. Family members with mental retardation
. . . . c. Family members with birth defects
. . . . d. Step parents

2. True/False: The risk of aneuploidy such as trisomy 21 only exists in women over 35 years old.

3. Increased paternal age is associated with which of the following:
. . . . a. Aneuploidy
. . . . b. Increased perinatal mortality and morbidity in otherwise normal fetuses
. . . . c. New dominant genetic mutations
. . . . d. Pregnancy medical complications

4. Midtrimester maternal serum screening utilized levels of these analytes (biochemical markers) except:
. . . . a. human chorionic gonadotropin
. . . . b. alpha-fetoprotein
. . . . c. fetal cortisol
. . . . d. unconjugated estriol

5. Potential confounding factors in the analysis of maternal serum screening include all of the following except:
. . . . a. Fetal demise
. . . . b. Wrong dates
. . . . c. Multiple gestation
. . . . d. Male fetus

6. Unexplained elevated maternal serum alpha-fetoprotein levels portends higher risk for the following perinatal outcomes except:
. . . . a. Oligohydramnios
. . . . b. Stillbirth
. . . . c. Gestational diabetes
. . . . d. Preterm delivery

7. In addition to the detection of aneuploid fetuses, maternal serum screening aids in all of the following except:
. . . . a. Detection of multiple gestations
. . . . b. Determining paternity
. . . . c. Detection of wrong estimation of gestational age
. . . . d. Identifying patients at risk for adverse perinatal outcome

8. Future maternal screening may involve the following analytes except:
. . . . a. Progesterone
. . . . b. Inhibin
. . . . c. Pregnancy Associated Placental Protein A
. . . . d. Urinary human chorionic gonadotropin core

9. True/False: The nuchal translucency measurement in the 10-13 week gestation as a predictor of aneuploidy is independent of maternal age.

10. Prenatal testing procedures currently include all of the following except:
. . . . a. Amniocentesis.
. . . . b. Fetal cells in the maternal circulation.
. . . . c. Chorionic Villus Sampling.
. . . . d. Percutaneous Umbilical Blood Sampling.


Answers to questions
1.d, 2.false, 3.c, 4.c, 5.d, 6.c, 7.b, 8.a, 9.true, 10. b


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