The "waters" or amniotic fluid is a crucial integral part of the pregnancy unit. There is a womb or uterus carrying the pregnancy, there is the fetus (baby), there is the placenta (afterbirth) and there is amniotic fluid, also known as "liquor". These are the four major components of the pregnancy unit.
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 plateauing, 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 below normal fluid volume, while "polyhydramnios", an equal mouthful, means above normal fluid volume.
In this chapter, 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 - if not most - cases, 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.
Where does the fluid around the baby come from?
This fluid is known as amniotic fluid. It is also called liquor.
As the pregnancy grows beyond the twelve-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.
What is the normal volume of fluid?
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 litre 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 forty-one or forty two weeks. It is important to emphasize that these figures are averages and that, at each mentioned stage, there is a range of volumes that will be considered perfectly normal.