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Can genetic or chromosomal abnormalities cause reduction of fluid volume?
Yes. Some chromosomal abnormalities are characterised by a chronically reduced fluid volume, almost always accompanied by fetal growth restriction. Because many of these abnormalities occur with other physical abnormalities, the diagnosis is likely to be suspected before delivery.
If this is the case, fluid could be taken from around the baby (by a procedure called amniocentesis) for analysis, to establish the diagnosis.
Does reduced fluid volume have any adverse effect on the growing fetus?
Yes. The most feared effect is poor lung development. Normal lung development depends on the presence of sufficient amniotic fluid. If the reduced volume starts very early and does not improve, lung development could be very poor and this is an extremely difficult problem to overcome.
The severity of this problem also depends on the degree of reduction in the fluid volume.
What are the other complications of low fluid volume?
The baby may be born with limb deformities, which are normally correctable. It is usually the feet that are affected (known as talipes or "club foot"). There is also an increased risk of fetal distress and delivery by caesarean section. As mentioned before, premature delivery - either spontaneous or induced - may be unavoidable.
What about rupture of the membranes?
Yes, this is also a cause of reduced amniotic fluid volume. In virtually all cases, this is immediately apparent. A complication unique to this is the risk of infection affecting the pregnancy. This significantly increases the risk of losing the baby. Labour tends to occur soon after rupture of the membranes, that is, within days.
However, it is not unknown for the pregnancy to continue for several weeks after rupture of the membranes.
Increased amniotic fluid volume
What are the probable causes of an increase in the amniotic fluid volume?
There are many known causes of abnormally high amniotic fluid volume. The most common ones include:
impaired fetal swallowing, because of obstruction in the gut or some kind of paralysis
increased urine output, which may result from twin-to-twin transfusion or a rare condition known as diabetes insipidus.
fetal heart failure - for instance, resulting from severe anaemia or a viral infection affecting the fetal heart.
So diabetes causes increase in fluid volume?
Yes, even though this is not in every case. Any woman who has an unexplained increase in fluid volume - usually in the last ten to fourteen weeks of pregnancy - will have a simple blood test to rule out diabetes. If excessive amniotic fluid occurs earlier in pregnancy, it is unlikely to be due to diabetes.
What could cause impaired fetal swallowing?
As mentioned before, fetal swallowing is the principle means of removing amniotic fluid from circulation. (The fluid is continually produced and removed, hence the term "circulation".)
If swallowing is impaired, there will be an imbalance, as the fluid is being produced but not removed.
Some babies have a developmental anomaly of gut obstruction. This means that although they can swallow, the fluid does not get anywhere. In other babies, the problem is neurological. If the baby has the severe brain anomaly called anencephaly - which in reality means there is no brain tissue - it will be unable to swallow. This condition is almost always recognizable early in pregnancy on the ultrasound scan and will almost certainly be picked up at the 18 to 20 weeks routine anatomy scan.