Amniocentesis and Amniotic Fluid Analysis

Norm of Amniocentesis and Amniotic Fluid Analysis

Routine Analysis
Color: Colorless, straw-colored, or clear to milky-colored.

SI Units
Acetylcholinesterase Negative
12 weeks of gestation ≤ 42 μg/mL
14 weeks of gestation ≤ 35 μg/mL
16 weeks of gestation ≤ 29 μg/mL
18 weeks of gestation ≤ 20 μg/mL
20 weeks of gestation ≤ 18 μg/mL
22 weeks of gestation ≤ 14 μg/mL
30 weeks of gestation ≤ 3 μg/mL
35 weeks of gestation ≤ 2μg/mL
40 weeks of gestation ≤ 1 μg/mL
Normal values may also be reported in multiples of the median (MOM) or 0.5–3.0 MOM.
Trimesters 1 and 2 ≤ 0.074 mg/dL ≤ 1.2 μmol/L
40 weeks of gestation ≤ 0.024 mg/dL ≤ 0.4 μmol/L
Calcium 4 mEq/L 4 mmol/L
Carbon dioxide 16 mEq/L 16 mmol/L
Chloride 102 mEq/L 102 mmol/L
≤27 weeks of gestation 0.8–1.1 mg/dL 71–97 μmol/L
30–34 weeks of gestation 1.1–1.8 mg/dL 97–159 μmol/L
35–40 weeks of gestation 1.8–4.0 mg/dL 159–354 μmol/L
Trimesters 1 and 2 ≤ 9 μg/dL ≤ 309 nmol/L
Term ≤ 59 μg/dL ≤ 2023 nmol/L
Glucose 30 mg/dL 2 mmol/L
<35 weeks of gestation 6–9 mg/dL
≥35 weeks of gestation 15–20 mg/dL
Lecithin/sphingomyelin (L/S) ratio
Immaturity ≤ 1:1 <1:1
Borderline maturity 1:1–2:1 1:1–2:1
Maturity >2:1 >2:1
After maturity ≥4:1 ≥4:1
Meconium Negative
Osmolality Equals serum osmolality
Trimesters 1 and 2 33–55 mm Hg 4.4–7.3 kPa
Term 42–55 mm Hg 5.6–7.3 kPa
Trimesters 1 and 2 7.12–7.38 7.12–7.38
Term 6.91–7.43 6.91–7.43
Potassium 4.9 mEq/L 4.9 mmol/L
Protein, total
Trimesters 1 and 2 0.36–0.84 g/dL 0.36–0.84 g/dL
Term 0.07–0.45 g/dL 0.07–0.45 g/dL
Sodium 7–10 mEq/L lower than serum sodium 7–10 mmol/L lower than serum sodium
Sphingomyelin 4–6 mg/dL
Total protein 2.5 g/dL 25 g/L
Trimesters 1 and 2 12–24 mg/dL 1.2–4 mmol/L
Term 19–42 mg/dL 3.2–7 mmol/L
Uric acid
Trimesters 1 and 2 2.76–4.68 mg/dL 0.17–0.28 mmol/L
Term 7.67–12.13 mg/dL 0.46–0.72 mmol/L


Abnormalities That May Be Found on Routine Analysis

Abnormal Color
Possible Cause
Yellow Caused by fetal bilirubin, erythroblastosis fetalis
Green Caused by meconium, breech presentation, fetal death, defecation, distress, hypoxia, intrauterine growth restriction, status post maturity, vagal stimulation
Red Caused by presence of blood, intrauterine hemorrhage
Port wine Acute fetal distress, abruptio placentae
Brown Oxidized hemoglobin, maternal tissue trauma, fetal death, fetal maceration


SI Units
Abnormal Bilirubin
Fetal involvement 0.10–0.28 mg/dL = 1 + 1.6–4.5 μmol/L
Later fetal involvement 0.29–0.36 mg/dL = 2 + 4.7–5.8 μmol/L
Fetal distress 0.47–0.95 mg/dL = 3 + 7.6–15.4 μmol/L
Fetal death >0.95 mg/dL = 4 + >15.4 μmol/L


SI Units
Abnormal Creatinine
35–40 weeks of gestation

Large muscle mass, possible diabetes

>4 mg/dL >354 μmol/L
Low birth weight <2 mg/dL <177 μmol/L


Increased Alpha-fetoprotein.
Anencephaly, cleft lip and palate, cystic fibrosis, duodenal atresia, esophageal atresia, fetal bladder neck obstruction with hydronephrosis, fetal death, meningomyelocele, multiple pregnancy, nephrosis (congenital), neural tube defects, spina bifida, omphalocele, and Turner's syndrome.
Increased Bilirubin.
Anencephaly, erythroblastosis fetalis, hemolytic disease of the newborn, hydrops fetalis, intestinal obstruction, and Rh sensitization.
Increased Lamellar Bodies in Amniotic Fluid.
Respiratory distress syndrome.
Positive Acetylcholinesterase.
Neural tube abnormalities that allow cerebrospinal fluid (which contains acetylcholinesterase) to leak into the amniotic sac.
Positive Meconium.
Fetal distress.
Decreased Alpha-fetoprotein.
Not applicable.
Decreased Bilirubin.
Not clinically significant.
Decreased Creatinine.
Fetal lung immaturity.
Chromosome Analysis.
Interpretation required.


Usage of Amniocentesis and Amniotic Fluid Analysis

Detection of fetal jeopardy or genetic disease and determination of fetal maturity.


Description of Amniocentesis and Amniotic Fluid Analysis

Detection of fetal jeopardy or genetic disease and determination of fetal maturity. Amniocentesis is a 20- to 30-minute procedure in which an aspiration of amniotic fluid is taken transabdominally and is usually performed after week 12 of gestation. In routine analysis, amniotic fluid is examined for levels of calcium, chloride, carbon dioxide, creatinine, estriol, glucose, pH, potassium, sodium, protein, urea, uric acid, and culture and for genetic defects, chromosomal studies, detection of fetal jeopardy or distress (by color, bilirubin), and to measure lung maturity (by L/S ratio) and age (by creatinine of the fetus). Alpha1-fetoprotein is a globulin protein secreted by the yolk sac and by fetal liver cells during hepatic cell multiplication. The highest amounts are found during pregnancy and in hepatic cancer. Measurement is usually performed from week 16 to 20 to help identify fetal neural abnormalities, gastroesophageal atresia, and nephrosis. Chromosome analysis of amniotic fluid cells is performed by examination of karyotyped cells for genetic abnormalities such as Down syndrome, Tay-Sachs disease, and other inborn errors of metabolism. Amniotic fluid is examined for color and bilirubin level for detection of fetal jeopardy or distress caused by hemolysis of fetal red blood cells. Erythroblastosis fetalis occurs when maternal antibodies attack fetal red blood cells, causing fetal anemia. This occurs when the mother's blood contains the Rh factor that reacts with fetal erythrocyte antigens. The test is usually performed at gestation week 24 or later and can help determine the need for intrauterine fetal blood transfusion. After the 35th week of pregnancy, the phospholipid levels of lecithin and sphingomyelin change in a predictable pattern that indicates the level of maturity of fetal lungs. Lecithin rises and sphingomyelin decreases as the fetal lungs mature.


Professional Considerations of Amniocentesis and Amniotic Fluid Analysis

Consent form IS required.

Bleeding, intrauterine death, premature labor, spontaneous abortion.
Abruptio placentae, incompetent cervix, placenta previa, and a history of premature labor.



  1. Obtain an amniocentesis tray, surgical scrub solution, a light-protected container, and povidone-iodine solution. Also obtain RhoGAM for Rh-negative mothers.
  2. Obtain maternal vital signs. Auscultate baseline fetal heart tones.
  3. Note the estimated date of conception and week of gestation on the laboratory requisition.
  4. Procedure should be performed in a darkened room if the specimen will be tested for bilirubin.
  5. See Client and Family Teaching.
  6. Just before beginning the procedure, take a “time out” to verify the correct client, procedure, and site.



  1. The position of the fetus and a pocket of amniotic fluid are determined using ultrasound and palpation, with the mother in a supine position.
  2. The mother's abdominal area is cleansed with surgical scrub solution and povidone-iodine and allowed to dry.
  3. The aspiration site is draped to demarcate a sterile field.
  4. The mother is instructed to place her hands behind her head, and the aspiration site is anesthetized with 1 mL of 1% or 2% lidocaine intradermally and subcutaneously.
  5. A 20- to 22-gauge, 5-inch-long spinal needle with a stylet is inserted through the abdominal wall into the intrauterine cavity, and the stylet is withdrawn.
  6. About 7–15 mL of amniotic fluid is aspirated through the spinal needle into a syringe, and the needle is withdrawn. Use a 20-mL amniotic fluid sample for direct genetic analysis for the four most common mutations responsible for Tay-Sachs disease.


Postprocedure Care

  1. Apply a dry, sterile dressing to the aspiration site.
  2. Inject 2–5 mL of amniotic fluid into a light-protected (foil-covered or amber) test tube to test for bilirubin. Inject 5–10 mL of amniotic fluid into a sterile, siliconized glass container or a polystyrene container for culture and genetic and other studies (AFP). Specimens to be transported to another site for testing should be packed in a cool, insulated container to maintain a temperature of 2–5 degrees C. Freezing temperatures should be avoided.
  3. Obtain the mother's vital signs. Auscultate fetal heart tones for changes from the baseline value.
  4. The mother should rest on her right side for 15–20 minutes after the procedure.
  5. RhoGAM may be prescribed for Rh-negative mothers.
  6. Transport the amniotic fluid specimen to the laboratory immediately and refrigerate.


Client and Family Teaching

  1. Empty your bladder immediately before the procedure if gestation is 21 weeks or more. You must have a full bladder during the procedure if gestation is 20 weeks or less.
  2. It is important to lie motionless throughout the procedure. You may experience a strong contraction with the needle insertion.
  3. Chromosome analysis results may take up to 4 weeks.
  4. After the procedure, notify the physician for cramping, abdominal pain, unusual vaginal drainage/fluid loss, fever, chills, dizziness, or more or less than the usual amount of fetal activity.
  5. Inform the client with abnormal genetic findings of choices regarding pregnancy and pregnancy termination. Also refer the client for genetic counseling before future attempts to become pregnant.


Factors That Affect Results

  1. Reject frozen or clotted specimens.
  2. Inadvertent aspiration of maternal urine can be ruled out by testing the specimen for blood urea nitrogen (BUN) and creatinine. Urine BUN is >100 mg/dL, whereas amniotic fluid is well under 100 mg/dL. Urine creatinine is usually >80 mg/dL, whereas amniotic fluid creatinine is usually ≤4 mg/dL.
  3. Nonsiliconized glass containers for routine analysis may result in cell adherence on the sides of the container.
  4. Amniotic fluid testing must be performed within 3 days of collection.
  5. Amniocentesis should be performed between weeks 24 and 28 when one is checking for hemolytic disease of the newborn and Rh sensitization.
  6. Falsely low bilirubin levels may result from failure to protect the specimen from light.
  7. Specimens contaminated with blood should be tested for fetal hemoglobin to determine whether the blood is of maternal or fetal origin. Fetal blood contamination results in falsely high bilirubin levels. Fetal or maternal blood will interfere with measurements of fetal lung maturity and amniotic fluid constituents that are also constituents of plasma, such as protein, potassium, and glucose.
  8. Creatinine levels are affected by maternal creatinine clearance and maternal creatinine levels. A concurrent maternal serum creatinine should be drawn. Maternal serum to amniotic fluid creatinine ratio should be about 2:1.
  9. Elevated AFP results may be caused by contamination of the specimen with fetal blood.
  10. Small and closed neural tube defects may not cause elevated AFP levels.
  11. Accurate L/S ratio measurement is not possible if the specimen is contaminated with blood (fetal or maternal) or meconium.


Other Data

  1. Direct karyotyping of placental villi samples obtained by needle aspiration has been found to yield faster results than amniotic fluid chromosome analysis. (See Chorionic villi sampling.)
  2. Chromosomal aberration has been found in 4.6% of fetuses in women >38 years of age, the most common being trisomy 21 (62%), Klinefelter's syndrome (11%), and Edward's syndrome (trisomy 18) (11%).
  3. For diamniotic twin pregnancies, each amniotic sac should be sampled.
  4. Early amniocentesis is feasible from 11 weeks of gestation and can be performed for the usual indications as an alternative to chorionic villus sampling. Results are available in less than 1 week using cytogenetic techniques.
  5. Prenatal cystic fibrosis profile may be performed by polymerase chain reaction (PCR) for mutations (F508, R553X, g551D, g542X, n1303K, and w1282X).
  6. Amniotic fluid neuron-specific enolase is useful as a marker for neonatal neurologic injury.