Increased amniotic fluid volume

There are many known causes of abnormally high amniotic fluid volume. The most common ones include:

v Diabetes

v Impaired fetal swallowing, because of obstruction in the gut or some kind of paralysis

v Increased urine output, which may result from twin-to-twin transfusion or a rare condition known as diabetes insipidus.

v Fetal heart failure - for instance, resulting from severe anemia or a viral infection affecting the fetal heart.

Diabetes and increased fluid volume (polyhydramnios)

Diabetes is a known cause of increased amniotic fluid volume, even though this is not in every case. Any woman who has an unexplained increase in fluid volume - usually in the last 10 to 14 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.

Impaired fetal swallowing

As mentioned before, fetal swallowing is the principle means by which amniotic fluid is removed 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 the gut and this is effectively obstructed. 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. Barring that, it will almost certainly be picked up at the 18 to 20 weeks routine anatomy scan.

There are other conditions that affect the muscles. The fetus is unable to effectively perform any activity requiring the use of muscles, including the act of swallowing. An infection by a virus called Coxsackie B which causes temporary paralysis is one example. It usually clears up in a couple of weeks or so. In the meantime, the mother will not feel any fetal movements at all.

Fetal heart failure

The heart is made up of muscle (myocardium) and, to function properly, it requires oxygen. If the amount of oxygen supplied to the heart muscle is insufficient, the heart will gradually fail. This can happen if the fetus is severely anemic. Such severe anemia may be caused by any condition destroying the baby's blood-cells.

Destruction of fetal blood cells might happen in a mother with a Rhesus negative blood group if she becomes sensitized. Such sensitization occurs when blood cells from a Rhesus positive baby find their way into the mother's circulation for the first time, where they trigger the production of antibodies against them.

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