(Lesson 3- Table of Contents) (Next) (Glossary)

Prenatal genetic services are offered in a number of different health care settings. MSAFP or the triple screen, for instance, is usually provided by a woman's primary physician. Many obstetricians perform genetic amniocentesis. Many pregnant women are also referred to centers that specialize in prenatal evaluation and counseling.

If a woman is referred to a prenatal genetics center she may meet with a genetic counselor who will usually obtain family history information, discuss the reasons for referral, explain the benefits and limitations of the proposed tests, and address other concerns that may arise. The actual procedures, such as chorionic villus sampling (CVS), amniocentesis or a detailed ultrasound study are performed by physicians and ultrasound technicians.

If the test results are normal, the information is communicated to the patient and/or the referring physician and routine care is recommended. If the test results are abnormal, follow-up counseling is provided by the prenatal genetics center staff.

The benefits of prenatal diagnosis vary. In most cases, parents will be reassured to learn that the fetus does not have the condition for which the test was being done. If a disorder is identified, the parents have several options. In rare instances, prenatal treatment may be available. Some parents choose pregnancy termination if the test results indicate that the fetus has a severe disability. Other couples choose to continue the pregnancy and prepare for the birth of their child, both medically as well as psychologically.

MATERNAL SERUM SCREENING

(Segments of this section were adapted from the article "Maternal Serum Screening for Chromosome Disorders and Open Neural Tube Defects" by Wendy Busch and Pat Himes which was published in Genetics Northwest, Volume 10, Number 2 & 3, December 1995. It is reprinted, with changes, with the permission of the Pacific Northwest Regional Genetics Group.)

Maternal serum alpha fetoprotein (MSAFP) testing is done to screen for open neural tube defects (NTDs). Anencephaly has an incidence of 1 in 2,000 and spina bifida 1 in 1,000, making NTDs one of the more common birth defects. Elevated levels of MSAFP are observed in 80-90% of affected pregnancies.

Elevated MSAFP levels have also been associated with chromosome abnormalities, omphalocele and gastroschisis (both with abdominal organs partly outside the body because of a defective abdominal wall), exstrophy of the bladder (open bladder), and rarely, congenital nephrosis of the kidney. Additionally, elevated MSAFP levels have been associated with an increased risk for adverse perinatal outcomes including pre-eclampsia, premature labor, low birth weight, placental abruption and stillbirth.

An association between low levels of MSAFP and Down syndrome (DS) was first observed in 1984. Using maternal age and MSAFP levels, it is estimated that 20-25% of fetuses with Down syndrome conceived by women under the age of 35 could be detected prenatally. With the addition of human chorionic gonadotropin (hCG) and unconjugated estriol testing (also called AFP3, Triple Screen, Multiple Marker, Prenatal Risk Profile, AFP+, etc.), the detection rate of fetuses with DS increases to approximately 60%. Maternal serum hCG levels are generally higher, while estriol levels are generally lower in pregnancies with DS. More recently, a placental hormone, dimeric inhibin, has been reported elevated in DS and, if included in the screening protocol, will increase the detection rate of DS to 80%. The association between trisomy 18 and lower levels of all three analytes is also well established.

The optimal time for maternal serum screening is between 15 and 18 weeks gestation. If the test results are normal, no further follow-up is required. If the test results are suggestive of an open neural tube defect or chromosome abnormality, and the gestational date is verified, further testing is recommended. Women with positive test results are offered the option of having an ultrasound examination to confirm the gestational age of the fetus and to identify any obvious structural defects or other factors, such as fetal demise or twins, that might explain the altered level of maternal serum markers. Should the cause of the abnormal MSAFP remain unresolved, women are given the option of having an amniocentesis for chromosome analysis and amniotic fluid AFP determination. If an NTD is suspected, the level of acetylcholinesterase will be determined. Acetylcholinesterase is a component of the fetal cerebrospinal fluid and, if found in the amniotic fluid, confirms the diagnosis of an open neural tube defect (Figure 3.1).

Keep in mind that the MSAFP or triple screen protocol is not intended to be a diagnostic test. A negative test result does not rule out the possibility that the fetus has DS, trisomy 18 or an open NTD. A negative result simply indicates that the risk is not increased relative to the general population. A positive screen indicates that the pregnancy is at a greater risk and further diagnostic testing needs to be offered. The MSAFP screening test is considered positive at 2.5 MoM, arbitrarily set to pick up 1 to 2% of women with elevated MSAFP levels. Only 1 in 15 women will be confirmed to have an elevated amniotic fluid AFP level.












Fig. 3.1. Steps in the maternal serum marker screen

ROUTINE PRENATAL DIAGNOSTIC PROCEDURES

(The following is adapted from the article "Prenatal Diagnosis Options" written by Pat Himes and published in Genetics Northwest, Volume 10, Number 2 & 3, December 1995. It is reprinted, with changes, with the permission of the Pacific Northwest Regional Genetics Group.)

There are currently several options available to pregnant women for the prenatal diagnosis of chromosome abnormalities, single gene disorders and multifactorial anomalies. These options include amniocentesis, early amniocentesis, chorionic villus sampling, and ultrasound examination.

CHORIONIC VILLUS SAMPLING

Chorionic villus sampling (CVS) is usually performed between 10 and 12 weeks gestation. Prior to the procedure most centers require a dating ultrasound and cervical cultures. The procedure is performed either transvaginally using a catheter or transabdominally using a needle. The device is inserted into the developing placenta under ultrasound guidance and the placental villi are aspirated into the attached syringe. The tissue is sent to the laboratory where the fetal villi are dissected from the maternal decidua and either processed directly or cultured for chromosomal, DNA or biochemical analysis.

From the mother's perspective, the transvaginal CVS is similar to a pelvic examination, except for a full bladder which is necessary for better ultrasound visualization. The aspiration is usually painless; however, the positioning (full bladder, vaginal speculum and abdominal pressure from the ultrasound transducer) is often uncomfortable.

Transabdominal CVS is similar to amniocentesis in that there may be discomfort and/or uterine cramping at the site of the needle insertion. Common complications following CVS include mild cramping and vaginal spotting in about 10% of patients. Most women feel well enough after CVS to resume normal activities. However, as with the other procedures, they are advised to avoid strenuous activities for 24-48 hours and to seek medical care in the event of more serious complications.

Potential risks associated with CVS include pregnancy complications which could result in miscarriage, inconclusive test results and increased risk for limb defects (i.e., missing fingers and toes or portions thereof) in the fetus. The risk for procedure-related pregnancy loss is slightly greater for CVS (1%) than amniocentesis. However, the exact risks are more difficult to ascertain due to the high background rate of pregnancy loss during the first trimester. Inconclusive results (e.g., placental chromosomal mosaicism, maternal cell contamination) may necessitate a second procedure. While controversial, the chance that a child will be born with craniofacial and limb defects is most likely <1%.

Limitations of CVS include the inability to perform a high resolution ultrasound at the early gestational age when CVS is performed and the inability to screen for neural tube defects. Some cytogenetic laboratories report that chromosomes from CVS are usually too short to identify microdeletions or subtle chromosomal abnormalities. Certain metabolic disorders are not expressed in villus cells, thus preventing prenatal diagnosis by CVS.

Benefits of CVS include the early gestational age at which the test can be done, providing early reassurance or the ability to make decisions about terminating the pregnancy at a time when the decision can be more private. DNA analysis for single gene disorders may have a shorter turn around time with CVS if the cells do not need to be cultured prior to analysis. Some metabolic disorders can also be diagnosed using CVS.

AMNIOCENTESIS

Amniocentesis is usually performed at 14-16 weeks gestation using a 22 gauge spinal needle to aspirate amniotic fluid. The needle is inserted through the abdomen just so it penetrates the amniotic sac. The procedure is done under concurrent ultrasound guidance so that there is continuous visualization of the needle. About 20-25 cc of amniotic fluid (fetal urine with fetal skin cells) is aspirated. Results are usually available in 10-14 days. In terms of discomfort, the procedure is generally comparable to a blood draw; although some women report uterine cramping. Generally, women resume their normal activities following amniocentesis. However, they are advised to avoid strenuous activity for 24-48 hours after the procedure, and to contact their physician in the event of complications.

The benefits of amniocentesis include a lower risk of significant pregnancy complications (<1/200) including risk of miscarriage. Amniocentesis also has the lowest risk for cell culture failure and maternal cell contamination. Amniotic fluid can be analyzed for alpha fetoprotein to diagnose neural tube and abdominal wall defects, or for infections. The fetal cells can be analyzed for chromosome abnormalities or biochemical disorders. Not only is the chromosome quality superior, but the amniotic fluid cells are more likely to represent the true fetal karyotype.

Routine amniocentesis is done early in the second trimester, about a month later than CVS. The amniocentesis procedure (i.e., a needle in the abdomen) can be very distressing to some women with needle anxiety. It may also take longer to receive the results of DNA testing if cultured cells are needed for analysis.

EARLY AMNIOCENTESIS

Early amniocentesis (EA) is performed prior to 14 weeks gestation. Each center may have specific criteria for performing the procedure such as evidence of chorioamniotic fusion, a posterior placenta, or absence of maternal obesity. Because not everyone will meet these criteria, women who are scheduled for an early amniocentesis are more likely to be rescheduled than women who are scheduled for routine amniocentesis or CVS. The procedure is identical to the routine amniocentesis procedure except that a smaller amount of fluid is extracted.

Risks involved with early amniocentesis are still not clearly defined. Published rates of pregnancy loss following EA range from <1% to 5.3%. There may be a higher rate of amniotic fluid leakage following EA. Some studies have suggested that the incidence of orthopedic and respiratory problems is higher than expected. Since a smaller quantity of fluid is withdrawn, fewer fetal cells are obtained and it may take longer to culture a sufficient number of cells for chromosome analysis.

Limitations of EA include the absence of norms for amniotic fluid alpha fetoprotein medians at early gestational ages, making it difficult to diagnose neural tube defects. A few enzyme-based metabolic disorders may also lack values for comparison at early gestational ages. Benefits of EA are similar to the benefits of CVS. The early gestational age at which the test is done enables early reassurance or early decision-making. The quality of the chromosome study is comparable to routine amniocentesis.

ULTRASOUND EXAMINATION

An ultrasound examination (US) is often performed for "routine" obstetric indications. It is used in conjunction with the above prenatal diagnostic procedures. It can also be used as the sole method of diagnosis for disorders that result in major fetal structural abnormalities. Some of the conditions that can be diagnosed prenatally using ultrasound include renal dysgenesis, congenital heart defects, skeletal dysplasias, hydrocephalus, polycystic kidney disease, gastroschisis, omphalocele, exstrophy of the bladder, distended bladder or stomach, NTD, hydrops, etc. Ultrasound is also used to monitor fetal health including breathing, body movements, fetal tone and umbilical blood flow.

Ultrasound detection of more than one major anomaly greatly increases the risk of an underlying chromosome abnormality. For instance, increased nuchal thickening or disproportionately short femur lengths and duodenal atresia are ultrasound findings that are seen more frequently in fetuses with Down syndrome. Choroid plexus cysts are associated with trisomy 18, cystic hygromas with Turner syndrome and holoprosencephaly with trisomy 13. When fetal structural abnormalities are observed, amniocentesis is recommended to rule out a chromosome abnormality. Amniocentesis should be done even if the pregnancy is farther advanced and precludes pregnancy termination. A normal fetal karyotype will encourage aggressive management and an abnormal fetal karyotype will suggest conservative follow-up of pregnancy.

SUMMARY

The decision to opt for prenatal diagnosis is often difficult and is influenced by a couple's feelings about the type of information that is provided through testing and the available options they feel are appropriate considering their moral, ethical and religious beliefs. No prenatal test can guarantee a healthy child and there is no treatment or cure available for the vast majority of conditions for which testing is possible. Structural defects, such as renal agenesis, may not be apparent on ultrasound examination when early testing (CVS or EA) is performed. Many conditions can be identified in the second trimester by detailed ultrasonography in conjunction with amniotic fluid alpha fetoprotein screening or fetal karyotyping.

The availability of prenatal diagnostic tests may be limited in your area, and testing options will change over time. It is for this reason that we recommend you establish a working relationship with the genetics or prenatal diagnostic clinic staff in your area. They will provide you with information regarding the availability of, and the risks associated with, specific prenatal tests. Genetics or prenatal center staff are also available to consult with your patients should they have additional questions or concerns.

PRACTICE ACTIVITY 1

Use a T or F to show whether each statement is true or false.

1. Women with high levels of MSAFP will have a baby with an open neural tube defect.

2. If a woman's triple screen is positive for Down syndrome, and the results of her amniocentesis show that she is carrying a chromosomally normal male fetus, routine follow-up is recommended.

3. List two limitations of CVS.

4. List two limitation of amniocentesis.

PRACTICE ACTIVITY 1: ANSWERS

1. False MSAFP (or the triple screen) is a screening test used to identify women who are at an increased risk of having children with an open NTD. These tests are not diagnostic. If a high level of alpha fetoprotein is found in a maternal serum sample, the gestational dates should be verified (an underestimated gestational age can lead to a falsely elevated MSAFP) and, if correct, an ultrasound examination/amniocentesis should be done.

2. False Any woman who has an abnormal triple screen and a normal ultrasound and amniocentesis is still at an increased risk for an adverse pregnancy outcome, including pre-eclampsia, premature labor, low birth weight, placental abruption and stillbirth.

3. Answers to this question may vary. The list of limitations could include the inability to perform a high resolution ultrasound study or screen for NTDs. The chromosomes may be too short to identify microdeletions or microduplications. It is not possible to diagnosis certain metabolic disorders using chorionic villi. The test results may be ambiguous due to placental mosaicism.

4. Answers to this question may vary. The list of limitations could include the fact that amniocentesis is performed in the second trimester. Some women are distressed by the procedure. It may take longer to receive test results and it may not be possible to test for certain metabolic disorders.

Lesson 3- Table of Contents) (Next) (Glossary)