Reduced, Increased (and other) amniotic fluid issues

The pregnancy unit is made up of four principal components: There is the womb (uterus), the baby, the placenta and the water (amniotic fluid). Occasionally there are changes that affect amniotic fluid and these could impact on the pregnancy as a whole as we shall see shortly.

In the majority of pregnancies, the amniotic fluid volume is normal. The volume changes according to the stage of the pregnancy. It tends to increase steadily throughout most of the course of the pregnancy, slowing down in the final ten weeks of pregnancy, peaking at about thirty-six weeks before leveling off, and then by thirty-eight weeks starting to go down again. This means that, at each stage of the pregnancy, there is a range of volumes that is considered normal. Any measurements that fall outside of this range are considered abnormal and may be a cause for concern.

Ultrasound scanning is the commonly available means of measuring the volume and tracking the trend of how the volume is changing. It is not perfect but is the best available tool for the job.

The term "oligohydramnios" means fluid volume below normal, while "polyhydramnios", an equal mouthful, means above normal fluid volume.

In this section, we have set out to explain the possible causes of these, how they are investigated, how the progress of pregnancy is monitored and the possible effects of each.

It is important to emphasize from the outset that in many the cause for the fluid volume abnormality is never found and that nothing untoward happens to the baby. Nonetheless, identified cases of abnormal fluid volume need to be investigated because, in those cases where the cause is identifiable, specific action may be imperative to ensure a successful pregnancy.

Source of amniotic fluid

As the pregnancy grows beyond the 12 weeks mark, fetal urination becomes the main source of amniotic fluid. The main means of the removal of the fluid is by the fetus swallowing it. This means that there is a continuous circulation of the fluid.

Normal amniotic fluid volume

At each stage of pregnancy, there is a range that is considered normal for that gestation.

When the estimation (usually by ultrasound scan) falls below the lower border of the range, the volume is considered subnormal. Likewise, if it is seen to be above the normal range, the doctor may arrange for some tests to establish the possible cause.

Any experienced midwife or doctor can detect increased amniotic fluid volume on examination alone. Reduced volume can also be suspected through feeling the abdomen, but is not as obvious.

As a very rough guide, total volumes are about 30 ml at ten weeks, 300 ml at twenty weeks, 600 ml at 30 weeks and about a liter (1000 ml) at thirty-eight weeks. Beyond this point, the volume gradually falls and will be about 700 ml at forty weeks and lower still if the pregnancy continues to 41 or 42 weeks. Of-course these are median levels and at each stage there is a range which will be considered perfectly normal.

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