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Oligohydramnios (too Little Amniotic Fluid)
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What is this and what is its significance?
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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.
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The main causes of oligohydramnios are:
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- 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.).
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What should be done if oligohydramnios is diagnosed?
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- 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.
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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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