The normal amount of amniotic fluid at term is approximately 1 litre. If the amount is thought to exceed 2 litres then a condition of poly-hydramnios is present. This is a potentially dangerous condition and occurs in about 3 per cent of all pregnancies.
Acute polyhydramnios is rare and invariably starts at the 24th week of pregnancy. There is a sudden, rapid enlargement of the uterus so that the abdominal girth may increase by as much as 15 or even 30 cm. in a few days. This is associated with a twin pregnancy and nearly always with a uniovular (identical) twin pregnancy. Rapid enlargement of the uterus may result in considerable abdominal discomfort or even pain. Should this happen the doctor must be notified.
The rapid uterine enlargement resulting from such a twin pregnancy usually continues for 7 to 10 days and then ceases. The pain and discomfort may be so severe that the woman has to be admitted to hospital for rest and special treatment. Premature labour may begin but the pregnancy usually continues satisfactorily and the twins, whose presence has been confirmed by X-ray, are delivered quite normally at the appropriate time.
Chronic polyhydramnios is usually first noted at about the 30th week of pregnancy, when the uterus enlarges to a degree greater than is expected. This is caused by the grossly increased amount of fluid within the uterus. The normal girth of a pregnant woman at term is approximately 100 cm. but when polyhydramnios occurs the girth at 34 weeks may reach 100 cm., and at term may reach as much as 115 or 120 cm.
Chronic polyhydramnios results in a slow enlargement of the uterus and is not associated with pain or discomfort until the uterus becomes so large that its size creates discomfort.
The causes of chronic polyhydramnios are:
2 Twin pregnancy.
5 Congenital abnormality of the foetus.
Unknown. Approximately 50 per cent of women suffering from polyhydramnios have no specific cause for the excessive amount of fluid present. The large quantity of fluid over-distends the uterus and predisposes the women to premature labour which is best avoided by rest and following the doctor’s advice meticulously. If the baby is in a large amount of amniotic fluid it may not assume its correct position so that delivery should be in hospital where any abnormality of the baby’s position can be corrected Women with polyhydramnios are treated with special care during labour. The contractions of the over-distended uterus are relatively inefficient so that labour may be prolonged and an assisted forceps delivery may be necessary during the second stage of labour. Weak uterine contractions during the third stage of labour may predispose the woman to bleeding after delivery, and she is usually given injections of ergometrine or syntocinon at the time of delivery rather than afterwards in order to prevent postpartum haemorrhage.
Polyhydramnios during pregnancy does not have a harmful effect on the development of the baby or on the woman after delivery, and there is no evidence to suggest that it will recur in a subsequent pregnancy.
Twin pregnancy. Acute polyhydramnios starts at 24 weeks in a twin pregnancy and chronic polyhydramnios usually develops at about the 28th or 30th week in a gradual manner. In actual fact twin pregnancies do not really suffer from polyhydramnios since there are two normal foetuses each with a normal amount of liquor, but the total amount of amniotic fluid is greater than normal and symptoms are exactly the same as for an excessive amount of fluid in a single pregnancy.
Diabetes. Diabetic women who are carefully and rigidly controlled throughout their pregnancy have only a slightly increased incidence of polyhydramnios. If diabetes is uncontrolled or poorly controlled in pregnancy there is a much higher incidence of polyhydramnios and the excessive amount of amniotic fluid is a direct result of the unstable diabetes.
Pre-eclampsia. Some women with polyhydramnios develop preeclampsia. The reason for this is not known, but it may be that there is an association between the over-distended uterus and the blood supply to the kidney.
Congenital abnormality. Polyhydramnios is one of the most important signs of some varieties of serious congenital abnormality.
Approximately 40 per cent of all single pregnancies which develop polyhydramnios do in fact harbour a baby suffering from a major degree of congenital abnormality. Polyhydramnios caused by congenital abnormality usually begins at about the 30th week and is indistinguishable from polyhydramnios resulting from other causes. Abnormalities of the central nervous system and of the spine of the baby may make its swallowing mechanism ineffective so that excess liquor accumulates within the amniotic sac. Abnormalities of the gullet, oesophagus or stomach may also result in defects of the swallowing mechanism and result in the formation of polyhydramnios.
Diagnosis of Polyhydramnios
Polyhydramnios may be diagnosed if the uterus is found to be overdistended by amniotic fluid when the estimated duration of pregnancy is known to be correct. It is one of the indications for abdominal X-ray to see whether the cause is twin pregnancy or a congenital abnormality of the foetus involving bone and will, therefore, be discernible by X-ray.
Treatment of Polyhydramnios
Polyhydramnios may cause gross abdominal distension, discomfort, clumsiness, and the uterus sometimes grows so large that there is swelling of the legs and shortness of breath. Sleeping may become a problem owing to the difficulty of getting into a comfortable position. Treatment is rest, and mild sedation may be required.
If the X-ray reveals that a baby has a congenital abnormality the pregnancy may be terminated by inducing premature labour. This, however, is a decision to be taken by the doctor who knows all the facts in each individual instance.
Amniocentesis is the removal of a small amount of amniotic fluid for examination. The investigation is completely painless. An area of skin on the anterior abdominal wall is treated with local anaesthetic and a needle is gently introduced into the cavity of the uterus whence a small quantity of amniotic fluid is withdrawn. This may be done early in pregnancy after the 12th week in order to diagnose Congenital abnormality (especially mongolism). It may be used later in pregnancy in the diagnosis of Rhesus incompatibility, for the diagnosis of other types of congenital abnormality, to estimate the baby’s maturity accurately and to indicate the condition of the baby’s lungs before inducing premature labour. In some instances of polyhydramnios, amniocentesis is performed to remove some fluid in order to relieve pressure within the uterus. It is not usually undertaken simply to establish the sex of the unborn baby by examining its cells.