Amniotic Fluid

Amniotic fluid is a clear, slightly yellowish liquid that surrounds the unborn baby (fetus) during pregnancy. It is contained in the amniotic sac. Amniotic fluid analysis is frequently associated with cytogenetic analysis. The clinical laboratory also performs several significant tests on amniotic fluid. Because amniotic fluid is a product of fetal metabolism, the constituents that are present in the fluid provide information about the metabolic processes taking place during—as well as the progress of—fetal maturation. When conditions that adversely affect the fetus arise, the danger to the fetus must be measured against the ability of the fetus to survive an early delivery.

Origin and Production The fluid is produced by the mother’s placenta during the first trimester and the early part of the second trimester, until the baby’s kidneys are mature enough to take over the task. The baby swallows the fluid as they “breathe” and then excretes it again as urine, thus maintaining the constant circulation of the fluid. However, the urine making up the fluid is not pure waste material as the majority of the fetal waste is passed through the placenta to be filtered by the mother’s kidneys.

Function The primary functions of the fluid are: To provide a protective cushion for the fetus, Allow fetal movement, Stabilize the temperature to protect the fetus from extreme temperature changes. To permit proper lung development. Exchanges of water and chemicals, also take place between the fluid, the fetus, and the maternal circulation. Helps with the uniform growth of the body parts and organs of the baby. Assists with the proper bone and muscle development

Volume The amount of amniotic fluid increases throughout pregnancy, reaching a peak of approximately 1 L during the third trimester, and then gradually decreases prior to delivery. The volume of amniotic fluid is positively correlated with the growth of fetus. From the 10th to the 20th week it increases from 25ml to 400ml approximately. From the 8th week, when the fetal kidneys begin to function, fetal urine is also present in the AF. Then the relationship between AF and fetal growth stops. It reaches the high peak of 800ml at the 28 week. The amount of fluid declines to roughly 400 ml at 42 weeks age.

An excessive amount of amniotic fluid is called polyhydramnios An excessive amount of amniotic fluid is called polyhydramnios. This condition can occur with multiple pregnancy (twins or triplets), congenital anomalies, or gestational diabetes. An abnormally small amount of amniotic fluid is known as oligohydramnios. This condition may occur with late pregnancies, ruptured membranes, placental dysfunction, or fetal abnormalities. Abnormal amounts of amniotic fluid may cause the health care provider to watch the pregnancy more carefully. Removal of a sample of the fluid, through amniocentesis, can provide information about the sex, health, and development of the fetus

Chemical Composition The placenta is the ultimate source of amniotic fluid water and solutes. Amniotic fluid has a composition similar to that of the maternal plasma and contains a small amount of sloughed fetal cells from the skin, digestive system, and urinary tract. The fluid also contains biochemical substances that are produced by the fetus, such as bilirubin, lipids, enzymes, electrolytes, nitrogenous compounds, and proteins that can be tested to determine the health or maturity of the fetus.

A portion of the fluid arises from the fetal respiratory tract, fetal urine, the amniotic membrane, and the umbilical cord. As would be expected, the chemical composition of the amniotic fluid changes when fetal urine production begins. The concentrations of creatinine, urea, and uric acid increase, whereas glucose and protein concentrations decrease. The concentrations of electrolytes, enzymes, hormones, and metabolic end products also vary (change) but are of little clinical significance.

Measurement of amniotic fluid creatinine has been used to determine fetal age. Prior to 36 weeks’ gestation, the amniotic fluid creatinine level ranges between 1.5 and 2.0 mg/dL. It then rises above 2.0 mg/dL, thereby providing a means of determining fetal age as greater than 36 weeks.

Specimen Collection Amniocentesis is recommended when screening blood tests such as the maternal serum alpha fetal protein test [AFP], human chorionic gonadotropin [hCG], and unconjugated estriol [UE3]), yield results that are abnormal. Fetal body measurements taken with ultrasonography accurately estimate the gestational age of the fetus and provide an assessment of the size and growth of the fetus throughout pregnancy

Finding an abnormality on the ultrasound could indicate potential fetal development problems and indicate the need for an amniocentesis and laboratory measurements of fetal lung maturity. Fetal epithelial cells in amniotic fluid indicate the genetic material of the fetus and the biochemical substances that the fetus has produced. These cells can be separated from the fluid, cultured, and examined for chromosome abnormalities by karyotyping, FISH, and DNA testing.

Storage Instructions: Freeze within 4 hours to transport to laboratory; stable refrigerated up to 1 week. Protect from light. Avoid repeated freezing and thawing of the specimen, which may cause the sample to precipitate, resulting in a lower than expected value. If cell culture is requested the specimen should be kept at 37oC.

Indications for Performing Amniocentesis Mother’s age of 35 or more at delivery Family history of chromosome abnormalities, such as trisomy 21 (Down syndrome). Parents carry an abnormal chromosome rearrangement Parent is a carrier of a metabolic disorder. Three or more miscarriages Fetal lung maturity

Collection Amniotic fluid is obtained by needle aspiration into the amniotic sac, a procedure called amniocentesis. The procedure most frequently performed is a transabdominal amniocentesis. Using continuous ultrasound for guidance, the physician locates the fetus and placenta to safely perform the procedure. A maximum of 30 mL of amniotic fluid is collected in sterile syringes. The first 2 or 3 mL collected can be contaminated by maternal blood, tissue fluid, and cells and are discarded. Fluid for bilirubin analysis in cases of hemolytic disease of the newborn (HDN) must be protected from light at all times.

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Can I Resume Normal Activities After an Amniocentesis? After an amniocentesis, it is best to go home and relax for the remainder of the day. You should not exercise or perform any strenuous activity, lift anything over 20 pounds (including children), and you should avoid sexual relations. You may take two Tylenol (acetaminophen) every 4 hours to relieve discomfort. The day after the procedure, you may resume all of your normal activities unless otherwise directed by your doctor. Call your doctor if you develop a fever or have bleeding, vaginal discharge, or abdominal pain that is more severe than cramps.

Color and Appearance Normal amniotic fluid is colorless and may exhibit slight to moderate turbidity from cellular debris, particularly in later stages of fetal development. Blood-streaked fluid may be present as the result of a traumatic tap, abdominal trauma, or intra-amniotic hemorrhage. The presence of bilirubin gives the fluid a yellow color and is indicative of red blood cell destruction resulting from HDN.

Differentiating Maternal Urine From Amniotic Fluid Differentiation between amniotic fluid and maternal urine may be necessary to determine possible premature membrane rupture or accidental puncture of the maternal bladder during specimen collection. Chemical analysis of creatinine, urea, glucose, and protein aids in the differentiation.

Levels of creatinine and urea are much lower in amniotic fluid than in urine. Creatinine does not exceed 3.5 mg/dL and urea does not exceed 30 mg/dL in amniotic fluid. Whereas values as high as 10 mg/dL for creatinine and 300 mg/dL for urea may be found in urine. Measurement of glucose and protein by a reagent strip is a less reliable indicator, because glucose and protein are not uncommon urine constituents during pregnancy.

Fetal Distress Clinical analysis of amniotic fluid assesses both fetal well being and maturation. Because amniotic fluid is a product of fetal metabolism, the constituents that are present in the fluid provide information about the metabolic processes taking place and the progress of fetal maturation. The oldest routinely performed laboratory test on amniotic fluid evaluates the severity of the fetal anemia produced by hemolytic disease of the newborn. Those infants are referred to as (Rh babies).

The incidence of this disease has been decreasing rapidly since the development of methods to prevent anti-Rh antibody production. The destruction of fetal red blood cells by antibodies that are present in the maternal circulation results in the appearance of the red blood cell degradation product, bilirubin, in the amniotic fluid. By measuring the amount of bilirubin present in the fluid, it is possible to determine the degree of hemolysis taking place and to assess the danger of this anemia presents to the fetus. In cases of premature or prolonged rupture of the amniotic membranes there is concern over possible infection of the mother and fetus.

Can amniotic fluid be tested to look for infections? Yes, there are a few tests that can be performed on amniotic fluid to detect infections that are passed from mother to baby during pregnancy (congenital infections). Some of these infections may have serious consequences for the developing fetus. A few examples include tests for cytomegalovirus (CMV), toxoplasmosis, and bacterial infections.