About seven out of every 100 pregnancies involves some form of problem with the amniotic fluid. Dr Howard Lee explains what happens when there is too little

What is amniotic fluid?
Amniotic fluid is usually sterile, and contained in the sac that surrounds your baby throughout pregnancy. It helps to protect and cushion, and also plays an important part in developing many of your baby's vital internal organs - like the lungs, kidneys and gut.

The normal amount may vary, but there's usually a slow increase until about 36 weeks of pregnancy - then a slow decrease. Most women carry about 500 ml of amniotic fluid. The fluid is produced by the inner lining of the sac (amnion) as well as a contribution from your baby's lungs and kidneys - it is continually taken up by the baby's swallowing, and is also sent across the afterbirth (placenta) into your own circulation.

The commonest problems with amniotic fluid - which occur in about seven out of every 100 pregnancies - usually take the form of either too much fluid (hydramnios or polyhydramnios) or too little fluid (oligohydramnios). Both conditions are associated with abnormalities in the baby's development or some other pregnancy complication. It has to be said that any abnormal difference in the amounts of the amniotic fluid may be either the cause or the result of a problem within a pregnancy.

Oligohydramnios
Too little amniotic fluid is much less usual than too much. It is more than possible for your baby to have too little fluid, and be born in perfect condition.

Causes and problems
These are often interrelated. Generally, too little fluid is caused by conditions that prevent - or reduce - the production of amniotic fluid. These include:

• Intrauterine growth restriction (poor growth of the baby) - which may even be adhesions of the baby's skin to the inner lining of the sac producing a resulting growth retardation.
• Various congenital (from birth) defects of the baby - especially kidney or urinary system malformations. There is a higher rate of limb abnormalities and congenital dislocation of the hip and talipes (foot abnormalities), possibly due to an increased pressure.
• Twin-to-twin transfusion syndrome.
• Post-mature (post-term) pregnancy: fluid reduction starts at about 36 weeks, and continues until delivery - a natural oligohydramnios, in other words.
• Premature rupture of the sac (membranes), with slow leaking; and thus a chronic oligohydramnios.

Other complications
Apart from the indications of possible birth defects in a baby, oligohydramnios can cause other, luckily rare, complications:

• As amniotic fluid is very important in the development of various organs in your baby - especially the lungs - too little fluid, especially for long periods of time, may cause these organs to develop abnormally or incompletely.
• Absence of the protective, cushioning function of the amniotic fluid may lead to possible intrauterine damage to your baby.
• In situations where there is a slow leaking of fluid, there is also an increased risk of infection developing within the amniotic sac.
• The biggest danger of a markedly reduced amount of amniotic fluid is complications at delivery; when there will be:
• Increased risk of compression of the baby's umbilical cord, thereby cutting off oxygen and nutrient supplies.
• Possible aspiration - breathing into the lungs - of thick meconium (baby's first bowel movement) during delivery.

• Decreased amount or lack of amniotic fluid noted on ultrasound examination; various 'index comparisons' are measured.
• Possible decreased movements of your baby - due to restriction in available space. Sometimes, for the same reason, increased awareness of movement may be noticed by some mothers-to-be.
• Apparent leaking of amniotic fluid, when the cause is a premature rupture of the sac.

• After major abnormalities of the baby have been excluded, an expedited delivery may be helpful if a mature enough stage of gestation is reached. In many other cases, close monitoring and frequent follow-up visits may be all that is necessary.
• Amnioinfusion - which is the transfusion of a special fluid into the amniotic sac to replace lost or low levels of amniotic fluid - may be offered or undertaken in some obstetric units. This is still, to some degree, in its experimental stage, but it may certainly help prevent the various lung abnormalities that can occur in the baby. It may also help to prevent compression of the baby's cord at delivery.