Multiple definitions of oligohydramnios are used because no ideal cutoff level for intervention exists.[1, 2, 3, 4, 5, 6, 7] Oligohydramnios is characterized by the following features:

Diminished amniotic fluid volume (AFV) Amniotic fluid volume of less than 500 mL at 32-36 weeks' gestation - Amniotic fluid volume depends on the gestational age; therefore, the best definition may be AFI less than the fifth percentile. Single deepest pocket (SDP) of less than 2 cm Amniotic fluid index (AFI) of less than 5 cm or less than the fifth percentile

See the images below.

Sonogram obtained before second-trimester amnioinfusion. This fetus has bilaterally absent kidneys consistent with a diagnosis of Potter syndrome. The cystic

structures in the renal fossae are most likely the adrenal glands. Sonogram obtained after second-trimester amnioinfusion. This fetus has bilaterally absent kidneys consistent with a diagnosis of Potter syndrome. The cystic structures in the renal fossae are most likely the adrenal glands. The earlier in pregnancy that oligohydramnios occurs, the poorer the prognosis. Fetal mortality rates as high as 80-90% have been reported with oligohydramnios diagnosed in the second trimester. Most of this mortality is a result of major congenital malformations and pulmonary hypoplasia secondary to PROM before 22 weeks' gestation. Midtrimester PROM (premature rupture of membranes) often leads to pulmonary hypoplasia, fetal compression syndrome, and amniotic band syndrome. The inspiration of amniotic fluid at regular intervals is probably needed for terminal alveolar development. The assessment of amniotic fluid volume is important in pregnancies complicated by abnormal fetal growth or IUGR. AFV has been shown to be predictive in discriminating normal from decreased growth. Oligohydramnios is a frequent finding in pregnancies involving IUGR and is most likely secondary to decreased fetal blood volume, renal blood flow, and, subsequently, fetal

urine output. Pregnancies complicated by severe oligohydramnios have been shown to be at increased risk for fetal morbidity.[8, 9] AFV is an important predictor of fetal well-being in pregnancies beyond 40 weeks' gestation. AFV must be closely monitored, with measurements obtained at least once per week. The diagnosis of oligohydramnios may help in identifying postterm fetuses in jeopardy. AFV is a predictor of the fetal tolerance of labor, and it is associated with an increased risk of abnormal heart rate, meconium-stained amniotic fluid, and cesarean delivery. An increased incidence of cord compression is associated with oligohydramnios; this can lead to variable decelerations, with cord occlusion as the proximate cause of fetal distress. Hydration (intravenous and oral) has been found to significantly improve the volume of amniotic fluid in third-trimester pregnancies with isolated oligohydramnios.[10] For more information, see Polyhydramnios and Oligohydramnios.

WorkupUltrasonographyThe diagnosis is confirmed by means of ultrasonography (see the images below). Oligohydramnios may be discovered incidentally during routine ultrasonography and noted during antepartum surveillance for other conditions. The diagnosis may be prompted by a lag in sequential fundal height measurements (size less than that expected for the dates) or by fetal parts that are easily palpated through the maternal abdomen.[11, 12, 13, 14, 15, 16] During ultrasonography of the fetal anatomy, normal-appearing fetal kidneys and fluid-filled bladder may be observed to rule out renal agenesis (see the following 2 images), cystic dysplasia, and ureteral obstruction. Check fetal growth to rule out intrauterine growth restriction (IUGR) leading to oliguria.

Sonogram obtained before second-trimester amnioinfusion. This fetus has bilaterally absent kidneys consistent with a diagnosis of Potter syndrome. The cystic

structures in the renal fossae are most likely the adrenal glands. Sonogram obtained after second-trimester amnioinfusion. This fetus has bilaterally absent kidneys consistent with a diagnosis of Potter syndrome. The cystic structures in the renal fossae are most likely the adrenal glands.

Sterile speculum examinationSterile speculum examination may be performed to check for range of motion (ROM). Amniotic fluid may pool in the vagina, and an arborization or ferning pattern may be observed when dried posterior vault fluid is examined microscopically. Cervical mucous may cause false-positive results, as can semen and blood. Nitrazine paper turns blue. The amniotic fluid is more basic (pH 6.5-7.0) than normal vaginal discharge (pH 4.5).

Measurement of amniotic fluid volumeInitial studies to objectively measure amniotic fluid volume (AFV) involved dye dilution techniques. The techniques were accurate, although they required amniocentesis, an invasive procedure that increased the risk of perinatal morbidity. The routine use of ultrasonography has created a safe, reliable, and repeatable method of measuring AFV. Early methods of assessing AFV with ultrasonography involved nonquantitative assessments, including sonographers' subjective impression of AFV. Subjective oligohydramnios criteria have included the following:

The absence of fluid pockets throughout the uterine cavity Crowding of the fetal limbs The absence of pockets surrounding the fetal legs Overlapping of the fetal ribs (in severe cases)

The 2 most commonly used objective methods of determining AFV include measurement of the single deepest pocket (SDP) and the summation of the SDPs in each quadrant, or the amniotic fluid index (AFI).[17] These tests are routinely performed with the patient in the supine or semiFowler position, although studies have demonstrated accuracy in the lateral decubitus position as well.[18, 19, 9, 20] The ultrasound transducer is held along the maternal longitudinal axis and maintained perpendicular to the floor while the SDP of the amniotic fluid is measured. Pockets should be free of fetal limbs and the umbilical cord, although some authors allow for a single loop of cord

to be within the fluid pocket. AFV may be artificially increased if the transducer is not maintained perpendicular to the floor. Excessive pressure on the maternal abdomen with the transducer may lead to an artificially reduced measurement (see the image below).

Amniotic fluid index (AFI) measurement technique. Phelan et al described the AFI as a quantitative measurement to predict a poor pregnancy outcome and the success of external cephalic versions.[1] The pregnant abdomen is divided into 4 quadrants by using the umbilicus as a reference point to divide the uterus into upper and lower halves and by using the linea nigra to divide the uterus into left and right halves. The 4 measurements are summed to obtain the AFI in centimeters.[19, 20] In gestations earlier than 20 weeks, measurements from the 2 halves are divided by the linea nigra to obtain the AFI. Tables of the normal limits for AFI, based on the gestational age, have been published for singleton and multiple pregnancies (see an example below). The mean AFI for normal pregnancies is 11-16 cm.

Amniotic fluid index (AFI) during a normal human singleton pregnancy. The solid line is the mean AFI, the lower dotted line is the 5th percentile value, and the upper dotted line is the 95th percentile value (data adapted from Moore, 1990). Image courtesy of Christopher L. Sistrom, MD. The test is reproducible, with interobserver and intraobserver variations of about 10-15% or 1-2 cm in pregnancies with normal AFVs. The margin of error is less in patients with decreased amounts of amniotic fluid. Oligohydramnios has been defined as an AFI less than 5 cm, although 8 cm has occasionally been used as a cut-off threshold. Because the AFV depends on the gestational age, oligohydramnios has been defined as an AFI less than the fifth percentile (corresponding to an AFI of < 6.8 cm at term).

Oligohydramnios has been defined as an SDP less than 2 cm. Perinatal morbidity rates have been shown to increase sharply with SDPs below this value. Some have suggested that an SDP of 2.53.0 cm is a better lower limit for separating normal SDPs from those consistent with oligohydramnios. Many studies have shown that the SDP and the AFI methods have equal diagnostic accuracies. The SDP technique may be a better means of assessing the AFV in twin gestations and in pregnancies at an early gestational age. Some study results have shown that the AFI has greater sensitivity and a higher predictive value than the SDP in diagnosing abnormally high and low AFVs. Most obstetricians prefer to assess a broader area of the uterine cavity by using the AFI because the single measurement of the SDP does not allow for an asymmetric fetal position in the uterus.

Other examinationsMRI and 3-dimensional (3D) ultrasonography are newer (and more expensive) modalities for accurately assessing the AFV.[21]

Amniotic wrinkleFinberg reported a possible pitfall in the sonographic analysis of amniotic fluid in twin pregnancies, the "amniotic wrinkle," which may give the misleading impression of adequate amniotic fluid for both twins when one twin actually has little to none. The author reevaluated sonograms of twin pregnancies in which an initial sonographer's assessment was adequate fluid for each twin but which the author's own imaging assessment on the same day showed oligohydramnios in one twin. He found either of the following may occur when oligohydramnios of one twin is present:

The intertwin membrane may fold in on itself, creating an amniotic wrinkle (a short linear structure that extends perpendicularly away from the twin with decreased amniotic fluid in toward the amniotic space of the other twin) An intrauterine sling or "cocoon" may be present, in which a fetus appears to be suspended within the amniotic space of the other twin may be present

Finberg recommended showing the intertwin membrane in all images used to document each twin's amniotic fluid, with additional right-angle images to identify amniotic wrinkles.[22]

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The amniotic fluid is part of the babys life support system . It protects your baby and aids in the development of muscles, limbs, lungs and digestive system. Amniotic fluid is produced soon after the amniotic sac forms at about 12 days after conception. It is first made up of water that is provided by the mother, and then around 20 weeks fetal urine becomes the primary substance. As the baby grows he or she will move and tumble in the womb with the help of the amniotic fluid. In the second trimester the baby will begin to breathe and swallow the amniotic fluid. In some cases the amniotic fluid may measure too low or too high. If the measurement of amniotic fluid is too low it is called oligohydramnios. If the measurement of amniotic fluid is too high it is called polyhydramnios.

What is oligohydramnios?Oligohydramnios is the condition of having too little amniotic fluid. Doctors can measure the amount of fluid through a few different methods, most commonly through amniotic fluid index (AFI) evaluation or deep pocket measurements. If an AFI shows a fluid level of less than 5 centimeters (or less than the 5th percentile), the absence of a fluid pocket 2-3 cm in depth, or a fluid volume of less than 500mL at 32-36 weeks gestation, then a diagnosis of oligohydramnios would be suspected. About 8% of pregnant women can have low levels of amniotic fluid, with about 4% being diagnosed with oligohydramnios. It can occur at any time during pregnancy, but it is most common during the last trimester. If a woman is past her due date by two weeks or more, she may be at risk for low amniotic fluid levels since fluids can decrease by half once she reaches 42 weeks gestation. Oligohydramnios can cause complications in about 12% of pregnancies that go past 41 weeks.

What causes low amniotic fluid?Birth defects Problems with the development of the kidneys or urinary tract which could cause little urine production, leading to low levels of amniotic fluid. Placental problems If the placenta is not providing enough blood and nutrients to the baby, then the baby may stop recycling fluid.

Leaking or rupture of membranes This may be a gush of fluid or a slow constant trickle of fluid. This is due to a tear in the membrane. Premature rupture of membranes (PROM) can also result in low amniotic fluid levels. Post Date Pregnancy- A post date pregnancy (one that goes over 42 weeks) can have low levels of amniotic fluid, which could be a result of declining placental function. Maternal Complications- Factors such as maternal dehydration, hypertension, preeclampsia, diabetes, and chronic hypoxia can have an effect on amniotic fluid levels.

What are the risks of having low amniotic fluid?The risks associated with oligohydramnios often depend on the gestation of the pregnancy. The amniotic fluid is essential for the development of muscles, limbs, lungs, and the digestive system. In the second trimester, the baby begins to breathe and swallow the fluid to help their lungs grow and mature. The amniotic fluid also helps the baby develop muscles and limbs by providing plenty of room to move around. If oligohydramnios is detected in the first half of pregnancy, the complications can be more serious and include:

Compression of fetal organs resulting in birth defects Increased chance of miscarriage or stillbirth

If oligohydramnios is detected in the second half of pregnancy, complications can include:

Intrauterine Growth Restriction (IUGR) Preterm birth Labor complications such as cord compression, meconium stained fluid and cesarean delivery

What treatments are available if I am experiencing low amniotic fluid?The treatment for low levels of amniotic fluid is based on gestational age. If you are not full tem yet, your doctor will monitor you and your levels very closely. Tests such as non-stress and contraction stress test may be done to monitor your babys activity. If you are close to full term, then delivery is usually what most doctors recommend in situations of low amniotic fluid levels. Other treatments that may be used include:

Amnio-infusion during labor through an intrauterine catheter. This added fluid helps with padding around the umbilical cord during delivery and is reported to help lower the chances of a cesarean delivery. Injection of fluid prior to delivery through amniocentesis. The condition of oligohydramnios is reported to often return within one week of this procedure, but it can aid in helping doctors visualize fetal anatomy and make a diagnosis.

Maternal re-hydration with oral fluids or IV fluids has shown to help increase amniotic fluid levels.

IntroductionA decreased amniotic fluid volume is frequently one of the first clues to an underlying fetal abnormality. The sonographer/sonologist should, therefore, have a basic understanding of the mechanisms responsible for normal amniotic fluid production. Once the derivation of amniotic fluid is understood, the potential mechanisms that can result in oligohydramnios can be better appreciated. Amniotic fluid has a number of important roles in embryo/fetal development: 1. Permitting fetal movement and the development of the musculoskeletal system. 2. Swallowing of amniotic fluid enhances the growth and development of the gastrointestinal tract. 3. The ingestion of amniotic fluid provides some fetal nutrition and essential nutrients. 4. Amniotic fluid volume maintains amniotic fluid pressure thereby reducing the loss of lung liquid - an essential component to pulmonary development. (Nicolini, 1989). 5. Protects the fetus from external trauma. 6. Protects the umbilical cord from compression. 7. It's constant temperature helps to maintain the embryo's body temperature. 8. It's bacteristatic properties reduces the potential for infection. The factors involved in regulating amniotic fluid volume are still not completely understood. The 6 proposed pathways (Brace, 1997) for fluid movement into and out of the amniotic cavity include: Pathway ml/day to the fetus 500-1000 170 ml/day to amniotic fluid 25 170 800-1200 10

Fetal swallowing Oral secretions Secretions from the respiratory tract Fetal urination Intramembranous flow across the placenta, umbilical cord 200-500 and fetal Transmembraneous flow from the amniotic cavity into the uterine circulation

Oligohydramnios DefinitionBrace and Wolf (1989) reviewed the literature and compiled 705 measurements of amniotic fluid between 8 and 43 weeks' gestation. All of the measurements were either by a dye-dilution technique or by direct measurement at hysterotomy. They reported that amniotic fluid volume

increased progressively until 33 weeks' gestation and then plateaued. The mean amniotic fluid volume between 22 and 39 weeks' gestation was 777 ml with the 95% confidence interval ranged from 302 ml to 1997 ml. This mathematical modeling of amniotic fluid volume provides a statistical definition of oligohydramnios. Whether this definition is clinically relevant with respect to fetal/neonatal outcome has not been determined.

Sonographic AssessmentThe sonographic assessment of amniotic fluid volume is semiquantitative. Historically, amniotic fluid volume was simply evaluated visually and graded as decreased, normal, or increased (Figure 1). The appearance of fetal crowding and an obvious lack of amniotic fluid were used to define oligohydramnios. Goldstein and Filly (1988) have reported good intra-observer and interobserver agreement between subjective assessment and the single largest pocket determination of amniotic fluid volume. One disadvantage of the subjective assessment of amniotic fluid volume is an inability to compare results from serial examinations as the fetal or maternal condition changes. Manning and Platt (1980) measured the single deepest pocket of amniotic fluid free of fetal extremities and umbilical cord to assess amniotic fluid volume (Figure 2).

Fig 1. Visually normal amniotic fluid volume at 18 weeks' gestation. Click for larger image.

Fig 2. 30 week gestation. A single deepest pocket of amniotic fluid (7 cm), indicating a normal amniotic fluid volume. Click for larger image.

Oligohydramnios was defined as the absence of a single pocket of amniotic fluid with a depth < 1.0 cm (Figure 3). This definition was found to be too restrictive. Manning redefined normal amniotic fluid volume as one pocket of amniotic fluid that measures at least 2.0 cm in two perpendicular planes (Manning 1995)(Figure 4).

Fig 3. Subjective assessment of amniotic fluid volume. 20 week fetus with a unilateral multicystic kidney (m) and congenital absence of the other kidney, resulting in anhydramnios (PL = placenta). Click for larger image.

Fig 4. 1.8 cm pocket of amniotic fluid indicating oligohydramnios. The color box confirms that umbilical cord is not present in the pockets of amniotic fluid. Click for larger image.

The amniotic fluid index (AFI) was proposed as a way to more fully assess the amount of amniotic fluid throughout the uterine cavity (Phelan, 1987a). This method summed the maximum vertical pocket of amniotic fluid in each quadrant of the uterus. Oligohydramnios was defined as an amniotic fluid index < 5.0 cm (Phelan, 1987a & b) (Figure 5A and B and Figure 6A and B)

Fig 5a. Upper quadrant. A normal amniotic fluid index of 19.6. The color box is used to ensure an absence of intervening umbilical cord. Click for larger image.

Fig 5b. Lower quadrant. Click for larger image.

Fig 6a. An amniotic fluid index of 4.2 cm, indicating oligohydramnios. a) there is an absence of amniotic fluid in the upper quadrants. The color box to the right of the image indicates the presence of umbilical cord. Click for larger image.

Fig 6b. There is a 2.5 cm ( C ) and 1.8 cm (A) pocket of fluid in the lower uterine quadrants. Click for larger image.

Moore and Cayle (1990) obtained AFI's in 791 normal pregnancies. They defined oligohydramnios as an AFI below the 5th percentile for gestational age. This value varied between 7.9 cm at 16 weeks and 6.3 cm at 40 weeks' gestation. Although an AFI of