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Oligohydramnios (too Little Amniotic Fluid)

What is this and what is its significance?

A mild degree of oligohydramnios causes no direct detrimental consequences to the fetus. However, severe oligohydramnios does have severe consequences for the fetus, irrespective of cause. In the uterus, the swallowing of water by the fetus promotes the normal development of the fetal lungs, and this is disrupted when there is a marked lack of amniotic fluid (severe oligohydramnios). In other words, chronic severe oligohydramnios may alone be sufficient to cause miscarriage, even when the cause is not known, because of its severe effect on fetal development.

In the first trimester of pregnancy, the main source of amniotic fluid is the mother, and a small amount originates from the fetus. In the second and third trimesters, most of the fluid is formed from fetal urine, and a decrease in the normal amount of amniotic fluid usually occurs as a result of a problem in the fetus's urinary tract.

The main causes of oligohydramnios are:

  • Most cases are a result of early rupture of the membranes and breaking of the waters. Sometimes there is a continuous, low-level loss of fluid that the mother does not even feel. Even slight leakage of amniotic fluid can be demonstrated using a special gynecological test.

  • Defects in the urinary tract and/or kidneys that do not allow urination in normal quantities. These can be congenital defects in the development of the kidneys or obstruction of the urinary tract at different levels (in the urethra, ureters, etc.).

  • Placental insufficiency. In this case, signs of delayed fetal growth also appear.

  • There are reports in the literature of a high frequency of chromosome disorders in fetuses with oligohydramnios. In most cases of oligohydramnios arising from chromosome disorders, other defects will also be detected in the fetus. Therefore oligohydramnios alone, without other defects, is not necessarily an indication for amniocentesis, especially if the cause is known (e.g. leakage of amniotic fluid, etc.).

What should be done if oligohydramnios is diagnosed?

  • The mother should be asked whether there has been any leakage of amniotic fluid in the period preceding the diagnosis of oligohydramnios. Her excretions should be examined for the presence of amniotic fluid, etc.

  • A 3rd level ultrasound scan should be performed to look for defects, especially in the urinary tract. In cases of severe oligohydramnios, a good fetal scan cannot always be performed.

  • Doppler testing of the placental blood vessels should be carried out in order to check for placental insufficiency.

  • If there is no clear cause, such as leakage of amniotic fluid or placental insufficiency, then the parents should be offered genetic counseling in order to discuss the severity of the condition, its implications, the need for further investigations, and the advisability of performing amniocentesis. In cases where amniocentesis cannot be performed, cordocentesis or chorionic villus sampling through the abdomen can be carried out instead.

In cases of severe oligohydramnios, with or without defects in the urinary tract, the parents should be referred to a multidisciplinary clinic in a hospital for professional counseling by an expert in pediatric kidney diseases (nephrologist), a genetic counselor, an expert in ultrasound examinations and a neonatologist in order to collate all the data and discuss what further investigations and interventions, if any, should be undertaken.

This will depend on the severity of the oligohydramnios, and whether or not there are any associated defects, etc.
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