Amniocentesis is an invasive procedure in which amniotic fluid is aspirated by means of a needle guided through the patient’s abdominal and uterine walls into the amniotic sac. Amniocentesis is preferably performed after the 15th week of pregnancy. By this time, amniotic fluid levels have expanded to 150 mL, so that a 10-mL specimen can be aspirated. If the purpose of amniocentesis is to ascertain fetal maturity, it should be done after the 35th week of 
gestation.

Amniocentesis provides a method to detect fetal abnormalities in situations in which the risk for an abnormality may be high. The test can evaluate fetal hematologic disorders, fetal infections, inborn errors of metabolism, and sex-linked disorders. It is not done to determine the sex of the fetus simply out of curiosity.

Chromosomal abnormalities and neural tube defects such as anencephaly, encephalocele, spina bifida, and myelomeningocele can be determined, as can estimates of fetal age, fetal well-being, and pulmonary maturity. Fluorescence in situ hybridization (FISH) technology is useful in the diagnosis of chromosomal abnormalities or deletion disorders. FISH can identify translocations, inversions, or deletions on chromosomes 13, 18, 21, and X and Y. This technique is most helpful if results are needed quickly for management of pregnancy.

The development of significant maternal Rh antibody titers or a history of previous erythroblastosis can be an indication for amniocentesis.

1. Position the patient on her back with her arms behind her head to prevent touching of the abdomen and the sterile field during the procedure (see Obstetric Sonogram in Chapter 13).

2. Perform ultrasound scanning before the procedure to assess fetal number, viability, and position. An appropriate pocket of amniotic fluid is localized in the scan. The tap site should be located away from the fetus, from the site of umbilical cord insertion, and from any thick placental segments.

3. Cleanse the skin thoroughly with an appropriate antiseptic solution and properly drape with sterile drapes. Inject a local anesthetic slowly at the puncture site.

4. Advance a 3.5- or 5-inch spinal needle (20- to 22-gauge) with stylet through the abdominal and uterine walls into the amniotic sac but away from the fetus and, when possible, from the placenta. Use continuous ultrasound surveillance to track the position of the fetus. Should the fetus move close to the needle, withdraw the needle.

5. Once the needle is properly positioned, remove the stylet and attach a syringe to the needle to permit aspiration of a 20- to 30-mL specimen. Discard the first 0.5 mL of aspirated fluid to prevent contamination by maternal cells or blood.

6. Withdraw the needle and place an adhesive bandage over the puncture site. Postprocedure ultrasound scanning confirms fetal viability.

7. Label specimen with the patient’s name, date, and test(s) ordered. Place the amniotic fluid specimen in a sterile brown or foil-covered silicone container to protect it from light and thereby prevent breakdown of bilirubin. Label the container properly. Include the estimated weeks of gestation and the expected delivery date. Deliver the sample to the laboratory immediately.

8. Be aware that the laboratory workup for genetic diagnoses usually takes 2–4 weeks to complete. However, specimens obtained for determination of fetal age (e.g., creatinine) take 1–2 hours; determinations of the lecithin-to-sphingomyelin (L/S) ratio and phosphatidylglycerol (PG) take 3–4 hours; Gram stain to rule out infection takes 30 minutes, and cultures take 48–72 hours.

9. The procedure may have to be repeated if no amniotic fluid is obtained or if there is failure of cell growth or culture results are negative.

10. Techniques have been developed for performing amniocentesis in the presence of twin fetuses. Amniotic fluid is aspirated from one of the amniotic sacs, and a small amount of contrast material is injected into the sac. When the adjacent sac is tapped and produces clear amniotic fluid, the healthcare provider is assured that each sac has been tapped and each fetus will be accurately assessed.

11. Record the type of procedure done, date, time, name of healthcare provider performing the test, maternal–fetal response, and disposition of specimen.

Amniocentesis is contraindicated in women with a history of premature labor or incompetent cervix and in the presence of placenta previa or abruptio placentae. Women who are carriers of hepatitis B or C should be informed of the limited information regarding transmission with invasive procedures. HIV-positive women should avoid invasive testing. If the amniotic fluid is bloody (blood is usually of maternal origin) and if a significant number of fetal cells (Kleihauer-Betke–positive smear) are present in the amniotic fluid of an Rh-negative patient, administration of RhoGAM should be considered. Some doctors prefer to administer RhoGAM to all Rh-negative patients for amniocentesis, unless they are already sensitized at that 
time.

1. Elevated amniotic fluid AFP can indicate possible neural tube defects as well as multiple gestations, fetal death, abdominal wall defects, teratomas, Rh sensitization, and fetal 
distress.

2. Decreased AFP is associated with fetal trisomy 21 (Down syndrome).

3. Creatinine levels are reduced in fetal prematurity. At 37 weeks of gestation, creatinine in amniotic fluid should be >2 mg/dL (>15 μmol/L).

4. Increased or decreased total AFVs are associated with certain types of arrested fetal development.

5. Increased bilirubin levels are associated with impending fetal death.

6. Amniotic fluid color changes are associated with fetal distress and other disorders such as chromosome abnormalities.

7. Sickle cell anemia and thalassemia can be detected through analysis of amniotic fibroblast DNA.

8. X-linked disorders are not routinely diagnosed in utero. However, because these disorders affect only men, the fetal sex may need to be determined when the patient is a known carrier of the X-linked gene in question (e.g., hemophilia, Duchenne muscular dystrophy).

9. Screening for carrier state or affected fetus is done through chromosomal testing.

10. The presence of some of the more than 100 detectable metabolic disorders can be detected in the amniotic fluid sample. Examples include Tay-Sachs disease, Lesch–Nyhan syndrome, Hunter syndrome, Hurler syndrome, and various hemoglobinopathies. Hereditary metabolic disorders are caused by absence of an enzyme due to deletion or by alteration of the structure or synthesis of an enzyme due to gene mutation. If the enzyme in question is expressed in amniotic fluid cells, it can potentially be used for prenatal diagnosis. An unaffected fetus would have a normal enzyme concentration, a clinically normal carrier of the gene defect would have perhaps half of the normal enzyme level, and an affected fetus would have a very small amount or none of the enzyme.

11. For disorders in which an abnormal protein is not expressed in amniotic fluid cells, other test procedures are necessary, such as DNA restriction endonuclease analysis.

Pretest Patient Care

1. Ensure that elective genetic counseling includes a discussion of the risk for having a child with a genetic defect and problems (e.g., depression, guilt) associated with selective pregnancy termination. The patient’s partner should be present and should share in the decision-making process. During genetic counseling, do not coerce the parents into undergoing pregnancy termination or sterilization; this should be an individual choice.

2. Explain test purpose, procedure, and risks; assess for contraindications.

3. Ensure that a properly signed and witnessed legal consent form is obtained.

4. Instruct the patient to empty her bladder just before the test.

5. Obtain baseline measurements of fetal and maternal vital signs. Monitor fetal signs for 15 minutes.

6. Alert the patient to the possibility that transient feelings of nausea, vertigo, and mild cramping may occur during the procedure. Help the patient to relax.

7. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Fetal blood contamination can cause false-positive results for AFP.

2. False-negative and false-positive errors in karyotyping can occur.

3. Polyhydramnios may falsely lower bilirubin values as a result of dilution.

4. Hemolysis of the specimen can alter test results.

5. Oligohydramnios may falsely increase some amniotic fluid analysis values, especially bilirubin; this can lead to errors in predicting the clinical status of the fetus.

Normal

• Normal amniotic fluid constituents and properties vary according to the age of the fetus and the laboratory methods used; pH is slightly alkaline. See descriptions of individual tests.