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Polyhydramnios (Excess Amniotic Fluid)

What is this and what is its significance?

Polyhydramnios is a subjective finding evaluated by the examining ultrasonographer. It occurs in 1% - 3% of pregnancies and is especially common in twin pregnancies. In most cases, it appears at the end of the second trimester or during the third trimester of pregnancy.

The common causes of polyhydramnios are as follows:

  • Maternal diabetes

  • A monozygotic twin pregnancy, as mentioned above. The appearance of polyhydramnios in the early weeks of gestation (18 - 24) of a twin pregnancy indicates either that the twins are identical (monozygotic), or that a pathological condition exists where blood is transferred from one fetus to the other (twin to twin transfusion). In this condition blood vessels develop that connect the two fetuses to each other, and this results in a situation where one fetus has a shortage of blood (anemia), and the other (the recipient) has a surplus (polycythemia). This condition can sometimes lead to several additional problems such as differences in growth and even heart failure in one of the fetuses.

  • Fetal defects - the frequency of associated defects in a fetus in the presence of polyhydramnios varies according to different reports. In cases of severe polyhydramnios, the risk for additional defects may reach a considerable level. The reported defects are: open central nervous system defects (neural tube defects), placental tumors, intestinal obstructions (especially of the esophagus), defects causing pressure on the fetal lungs (skeletal disorders, pulmonary cysts, etc.), defects in the fetus's limbs causing reduced motility, and diseases associated with muscular problems such as myotonic dystrophy, etc.

  • Chromosome disorders - there are various reports on the relationship between polyhydramnios and chromosome disorders. When additional defects are diagnosed, the risk that there is an associated chromosome disorder increases significantly, but even in cases where marked polyhydramnios occurs as an isolated finding, where no obvious cause has been found (such as diabetes or twins), there is approximately a 5% risk of an associated chromosome disorder. Recent studies have shown a lesser rate of chromosomal anomalies in cases of polyhydramnios if no other abnormalities are found in the fetus.

What should be done if polyhydramnios is diagnosed?

  • The mother should be investigated for diabetes by performing a glucose tolerance test.

  • Ultrasound examination should be performed to look for fetal defects and repeated in order to monitor the quantity of amniotic fluid. In cases where the quantity increases and there is a risk that the mother may go into early labor or develop breathing difficulties, the physician may consider more active intervention. The same applies in cases of identical (monozygotic) twin pregnancies.

  • The parents should be referred for genetic counseling in order to receive a full explanation of the condition, its significance and implications, whether further investigations should be performed, and to discuss the question of amniocentesis. The "gold-standard" threshold for recommendation of amniocentesis is a risk of Down syndrome greater than 1:386 based on the results of the biochemical marker screening tests, and when the weighted risk is equal to or higher than this amniocentesis is generally recommended. However, the presence of polyhydramnion in the fetus may statistically increase the risk of Down syndrome. Therefore in these cases some physicians suggest that this should be integrated with the results of the biochemical screening tests and the new threshold for recommending amniocentesis is a risk for Down syndrome of greater than 1:1000 in the biochemical marker screening tests.

  • It is especially important to offer genetic counseling when there is no obvious reason for the polyhydramnios, or when the polyhydramnios is very marked or other defects are also present.

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