Polyhydramnios Case File

Polyhydramnios Case File

Eugene C. Toy, MD, Edward Yeomans, MD, Linda Fonseca, MD, Joseph M. Ernest, MD

Case 43

A 36-year-old primigravida at 28 weeks’ gestation is seen in your office for a routine prenatal visit. She has had an uncomplicated pregnancy to date including normal maternal serum screening at 16 weeks’ gestation. Ultrasound at 18 weeks’ gestation revealed an appropriately grown

singleton infant with no obvious anatomic anomalies and normal amniotic fluid volume. She currently reports normal fetal activity and rare uterine contractions since her last visit in your office. She denies vaginal leaking or bleeding. She does feel that she has grown much larger over the past 4 weeks than in comparable 4-week intervals during this pregnancy, and says that when she is at work, many people think she is “due any day” because of her increasing abdominal girth.

On physical examination, she is alert and oriented, and states she feels well. Her blood pressure is 120/80 mm Hg and her weight is 138 lb, which is up 6 lb over the past month. Her fundal height is 36 cm. Ultrasound in your office reveals a vertex fetus with estimated fetal weight of 1500 g, normal head circumference/abdominal circumference ratio, and amniotic fluid index of 26 with single deepest vertical pocket of 10 cm.

➤ What is the most likely diagnosis?

➤ What is your next step?

➤ What are potential complications of the patient’s disorder?

ANSWERS TO CASE 43:

Polyhydramnios

Summary: This is a 36-year-old at 28 weeks with a rapidly enlarging fundal height due to excessive amniotic fluid volume.

➤ Most likely diagnosis: Hydramnios (or polyhydramnios).

➤ Next step: Evaluate fetal anatomy, test for maternal diabetes, and monitor for preterm labor.

➤ Potential complications: Preterm labor, preterm PROM and abruptio placenta, fetal anomalies, maternal diabetes, abnormal fetal presentation, postpartum hemorrhage.

ANALYSIS

Objectives

1. Describe the production and regulation of amniotic fluid.
2. List risk factors for oligo- and polyhydramnios.
3. Explain fetal anatomic reasons for abnormal amniotic fluid volume measurements.
4. List maternal reasons for abnormal amniotic fluid volume measurements.
5. Discuss management of the pregnancy with excess or inadequate amniotic fluid volume.

Considerations

This patient represents a commonly seen issue in obstetrics—an unexplained increase in amniotic fluid in the third trimester. Although this patient’s ultimate etiology may be as simple as maternal hyperglycemia, and may indicate little or no excessive fetal or maternal morbidity, half or more of cases of polyhydramnios remain idiopathic with no obvious cause discovered even after birth. This case should include maternal glucose screening, repeat ultrasound evaluation for anomalies that cause polyhydramnios and an estimation of fetal weight, and assessment for uterine contractions and prodromal preterm labor. If the workup is negative, follow-up ultrasound in 2 to 3 weeks would be the next step in her care.

APPROACH TO

Polyhydramnios

Normal Amniotic Fluid

Although term human fetuses vary in size, an average 3.5 kg fetus contains about 2500 mL of water, 350 mL of which is intravascular and 1000 mL of which is intracellular, the remainder being extracellular. The placenta contains about 500 mL of water, and 500 to 1200 mL of water is present in the amniotic fluid. Amniotic fluid functions include those of a physical nature to protect the fetus and cord, a functional nature to allow substrate for breathing, movement, and swallowing activities, and a homeostatic nature to protect against infection and regulate uterine activity.

Amniotic Fluid Dynamics

During the first trimester, amniotic fluid is isotonic with maternal or fetal plasma and represents a transudate of plasma, either from the fetus across nonkeratinized fetal skin or from the mother across the uterine decidua and/or placental surface. With advancing gestation, amniotic fluid composition diverges from that of plasma as the transudative process becomes negligible by 22 to 23 weeks’ gestation. Amniotic fluid osmolality and sodium composition decrease, felt to be the result of dilute fetal urine. During the second half of pregnancy, in keeping with the increasing fetal urine production, amniotic fluid urea, creatinine, and uric acid increase resulting in amniotic fluid concentrations of urinary by-products two to three times higher than fetal plasma. At this time, amniotic fluid is formed primarily from fetal lung fluid (about 100 mL/d) and fetal urine (7-10 mL/kg/h) and is eliminated primarily by fetal swallowing (up to 1 L/d). Fetal lung fluid production is affected by a diversity of fetal physiologic and endocrine factors including AVP (amniotic vertical pocket), catecholamines, and cortisol (all of which decrease lung fluid production), effects that may explain the enhanced clearance of lung fluid in fetuses delivered after labor compared to elective cesarean section.

The amount of fluid swallowed by the fetus daily does not equal the amount of fluid produced by the sum of kidneys and lungs, and because the volume of amniotic fluid does not greatly increase during the last half of pregnancy, intramembranous transfer of fluid is suspected to maintain this equilibrium. The intramembranous pathway refers to the route of absorption from the amniotic cavity directly across the amnion into fetal vessels (as opposed to the transmembranous pathway which refers to absorption from amniotic fluid to maternal blood within the uterus). In sheep, the intramembranous pathway, combined with fetal swallowing, approximately equals the flow of urine and lung liquid under homeostatic conditions. In humans, indirect evidence supports the existence of a similar pathway. This intramembranous pathway may explain the observation that in upper GI tract obstruction in the human fetus with no fetal swallowing, only 40% develop polyhydramnios.

Amniotic Fluid Volume Measurements

In invasive studies using amniocentesis to instill dye with remeasuring after mixing, neither single maximum vertical pocket (MVP) nor the four quadrant amniotic fluid index (AFI) produce very accurate evaluation of actual volume. Since clinical correlations are established between these ultrasound estimates and clinical outcome, however, the absolute volume present is not a primary issue. Categorization of polyhydramnios by MVP is more closely associated with clinical outcome, and in management comparisons reported in the Cochrane database in 2008, neither MVP nor AFI was superior in preventing admission to the NICU, an umbilical artery pH less than 7.1, the presence of meconium, an APGAR score of less than 7 at 5 minutes, or cesarean delivery. However, the AFI was associated with significantly more diagnoses of oligohydramnios and more women had inductions of labor and cesarean sections for fetal stress. The authors concluded that the MVP was the preferred method to assess AFV during fetal assessment, since the AFI increased the diagnosis of oligohydramnios and intervention without improvement in peripartum outcomes.

Oligohydramnios

Reduced amniotic fluid has been linked with numerous adverse pregnancy outcomes, and thus its detection is critical. Subjective assessments of low amniotic fluid volume have been replaced by quantitative measures of either MVP (also referred to as single deepest pocket, or SDP) or amniotic fluid index (AFI). Both of these measurements were introduced in the literature in the 1980s, with the AFI initially being promoted as the more accurate method to assess fluid volume. In an attempt to confirm previously reported normative data with both techniques, however, Magann et al1 reported that when using an AFI of < 5 cm or SDP of less than 2 cm as criteria for oligohydramnios, 8% of normal gravidas were falsely labeled as having oligohydramnios compared to only 1% with the SDP. In reviewing randomized clinical trials comparing AFI to SDP, using the SDP to detect oligohydramnios was superior and resulted in fewer unnecessary interventions with similar fetal outcomes. The SDP is also the method of amniotic fluid assessment used in the biophysical profile (BPP). Thus, he and others recommend the SDP less than 2 cm as the most appropriate method to detect oligohydramnios.

Oligohydramnios prior to 20 weeks gestation is rare, and should prompt a careful search for renal agenesis, bladder outlet obstruction, another significant fetal anomaly, or rupture of the fetal membranes. After 20 weeks, additional reasons to be considered are inadequate prerenal volume (in situations

such as IUGR, twin-twin transfusion syndrome, or unexplained markedly elevated MS-AFP which may indicate placental dysfunction and high perinatal mortality rates). Uterine and maternal etiologies should be sought, including intravascular dehydration or preeclampsia, and a careful search for PROM should occur as well in all cases of oligohydramnios after 20 weeks. When borderline oligohydramnios is suspected, maternal hydration should be considered, as improvements in AFI have been noted after IV or PO fluid administration in several studies. 

Isolated oligohydramnios in the third trimester when other fetal testing is reassuring is an area of ongoing debate. In a recent national survey of perinatologists, 92% of respondents felt isolated oligohydramnios represented a potential precursor of adverse pregnancy outcome, but only 33% felt induction of labor could decrease adverse outcomes.

Polyhydramnios

Excessive amniotic fluid complicates up to 4% of pregnancies, usually being associated with a normal neonatal outcome. However, as Table 43–1 indicates, the greater the degree of excess fluid, the more likely an adverse outcome becomes. Over 50% of patients with polyhydramnios have no identifiable cause, and when detected in the third trimester or when associated with macrosomia, usually have a good outcome. When detected in the second trimester or when associated with low or normal birth weight, however, nearly 30% of infants will have an abnormality detected during or after the first year of life, and experts recommend in those cases, careful antenatal ultrasound evaluation of fetal movement (to exclude hypotonia), and the fetal face and heart as possible causes of the hydramnios. 

When an antenatal cause is identified, it usually is the result of maternal diabetes, multiple gestation, GI tract obstruction, underlying neurologic disorders that prevent or reduce fetal swallowing, cardiac anomalies (structural, functional, or rhythm disturbances), or chromosomal or genetic disorders.

Data from Harman CR. Amniotic fluid abnormalities. Semin Perinatol. 2008;32:288-294.

SDP = Single deepest vertical pocket; AFI = amniotic fluid index; PNM = perinatal mortality

Structural fetal anomalies associated with hydramnios have an associated 9% to 10% risk of aneuploidy, compared to the 1% risk of aneuploidy with isolated hydramnios, and patients should be counseled regarding amniocentesis accordingly. Rh isoimmunization, metabolic disorders, and fetal infections may also cause hydramnios and should be considered prior to categorizing the etiology as idiopathic. 

Management of the patient with unexplained hydramnios includes evaluation for preterm labor, preterm rupture of membranes and abruptio placenta, abnormal presentation in labor, and postpartum hemorrhage. At delivery, the infant’s pediatrician should be notified of the history of hydramnios so a thorough evaluation of the neonate can occur.

Comprehension Questions

43.1 Complete bladder outlet obstruction viewed sonographically at 16 weeks’ gestation would be associated with which of the following?

A. Severe oligohydramnios

B. Normal amniotic fluid volume

C. Polyhydramnios

D. Unable to determine

43.2 Production of amniotic fluid includes which of the following organ systems?

A. lungs

B. liver

C. bone

D. CNS

43.3 Oligohydramnios is best detected clinically with few false positives using which of the following techniques?

A. AFI

B. SDP

C. Subjective assessment

D. Dye dilution techniques

43.4 Polyhydramnios, when idiopathic, is associated with neonatal abnormalities in which of the following ranges?

A. 10%

B. 20%

C. 30%

D. 40%

ANSWERS

43.1 B. Fetal skin keratinization occurs around mid-gestation, and at 16 weeks, complete bladder outlet obstruction would appear sonographically with normal amniotic fluid volume.

43.2 A. In addition to fetal urination, the fetal lungs produce up to 100 mL of amniotic fluid daily.

43.3 B. Diagnosing oligohydramnios using the single deepest pocket measurement is associated with fewer false positives, and similar perinatal outcomes to the AFI calculation.

43.4 C. Idiopathic polyhydramnios detected prior to the third trimester and not associated with macrosomia is associated with neonatal abnormalities in 28% or more of infants.

Clinical Pearls

See US Preventive Services Task Force Study Quality levels of evidence in Case 1

➤ Amniotic fluid is isotonic with fetal and maternal plasma prior to 22 weeks’ gestation (Level II-3).

➤ In addition to fetal swallowing and lung and urinary production, the intramembranous pathway regulates amniotic fluid volume (Level II-2).

➤ The single deepest pocket (SDP) is superior to the amniotic fluid index (AFI) when evaluating for oligohydramnios, and results in fewer false-positive results (Level II-1).

➤ Idiopathic polyhydramnios has an up to 28% chance of neonatal abnormality, and should prompt a careful newborn examination (Level II-3).

REFERENCES

1. Magann EF, Chauhan SP, Doherty DA, Magann MI, Morrison JC. The evidence for abandoning the amniotic fluid index in favor of the single deepest pocket. Am J Perinatol. 2007;24:549-558. 

2. Beall MH, van den Wijngaard JPHM, van Gemert MJC, Ross, MG. Amniotic fluid water dynamics. Placenta. 2007;28:816-823. 

3. Dashe JS, McIntire DD, Ramus RM, Santos-Ramos R, Twickler DM. Hydramnios: anomaly prevalence and sonographic detection. Obstet Gynecol. 2002;100:134-139. 

4. Dorleijn DMJ, Cohen-Overbeek TE, Groenendaal F, Bruinse HW, Stoutenbeek P. Idiopathic polyhydramnios and postnatal findings. J Matern Fetal Neonatal Med. 2008 Dec;13:1-6. 

5. Harman CR. Amniotic fluid abnormalities. Semin Perinatol. 2008;32:288-294. 

6. Nabhan AF, Abdelmoula YA. Amniotic fluid index versus single deepest vertical pocket as a screening test for preventing adverse pregnancy outcome. Cochrane Database Syst Rev 2008; 3. Art. No.:CD006593. doi: 10.1002/14651858.CD006593.pub2. 

7. Schwartz N, Sweeting R, Young BK. Practice patterns in the management of isolated oligohydramnios: a survey of perinatologists. J Matern Fetal Neonatal Med. 2008 Dec;16:1-5.

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