Throughout its whole development and growth the baby is surrounded by the amniotic fluid, otherwise known as the liquor amnii or just simply liquor. It is contained by the amniotic membrane within the amniotic cavity, and is formed very early in the development of the human embryo. The amniotic fluid gradually changes its constituents as pregnancy progresses.
In early pregnancy the size of the amniotic cavity is larger than is required by the small foetus, therefore affording the growing foetus complete freedom to move and develop.
At about the 16th week of pregnancy the baby has complete freedom to move, turn and rotate but can now be palpated by the skilled examiner through the wall of the uterus. This freedom continues until about the 30th week of pregnancy, when the baby has a tendency to become fixed in a longitudinal position within the uterine cavity. In some pregnancies, however, there is still sufficient fluid even at 34 weeks to allow the baby to rotate or turn quite freely.
Approximately 1 litre of amniotic fluid is present at the 36th week of pregnancy, after which the amount of fluid gradually diminishes until the expected date of delivery is reached, when the amount present is usually just under 1 litre. If delivery does not occur at the 40th week the quantity of amniotic fluid is reduced more rapidly, so that by the 42nd week 100 or 200 ml. may be all that is remaining.
Functions of Amniotic Fluid
1. Amniotic fluid provides a liquid environment in which the foetus can develop and exercise in absolute freedom.
2. It causes enlargement of the amniotic cavity and, therefore, of the uterus, so that the developing foetus is not distorted by pressure from the wall of the uterus.
3. It provides an environment of constant temperature.
4. Its constituents vary as pregnancy advances and as the requirements of the foetus change.
5. It provides a means of excretion of substances from the foetus, especially urine.
6. The foetus also swallows liquor. It is not known how much nutritional benefit this may have for the foetus, but it enables it to develop its mouth, as well as its swallowing and breathing mechanisms.
7. The amniotic fluid functions as a shock absorber which is most efficient in preventing damage to the foetus as a result of a blow or undue pressure. If undue force is brought to bear upon any particular part of the uterus then the placenta may suffer damage, or may even be separated, but it is virtually impossible to damage the foetus itself as a result of direct or indirect injury.
Many women fear that their baby will be damaged by a fall or a direct blow upon the uterus or abdomen during pregnancy. If a pregnant woman does suffer from direct injury, or if she falls down on her abdomen, she is unlikely to have caused injury to her pregnancy but she should consult her doctor if she develops pain or if she is unduly concerned. Falling down stairs, tripping over the edge of a carpet or pavement, and motor-car accidents are the three most common causes of abdominal injury during pregnancy. Contrary to some of the ancient legends, injury to the abdomen will not cause deformity or congenital abnormalities of the foetus. Obviously pregnancy does not prevent bones being broken or other injuries occurring, but direct injury to the baby itself is very unlikely.
Origin of Amniotic Fluid
The method of formation, disposal and of the control of amniotic fluid are not really understood. It is probably formed from the cells which line the amniotic membrane and during the second half of pregnancy the foetus urinates into the amniotic fluid. Some of the amniotic fluid is probably absorbed either into the umbilical cord or into the cells’lining the amniotic membranes, and during the second half of pregnancy the foetus swallows some amniotic fluid. The amniotic fluid is formed by tissues of foetal origin and not of maternal origin. It is secreted and absorbed continuously.
Leakage of Amniotic Fluid
When the membranes rupture during labour amniotic fluid leaks out of the uterus and down the vagina. Rupture of the membranes before the onset of labour is inevitably followed by labour either prematurely or at term. In certain instances leakage of amniotic fluid precedes miscarriage. When the membranes rupture there is a sudden rush, or gush, of liquor which is unmistakable. Should this happen the pregnant woman must go to bed immediately and notify her doctor or midwife, or telephone the hospital.
Sudden rupture of the membranes with the loss of a large amount of liquor is always obvious, but there may occasionally be a small leakage of liquor which is much more difficult to diagnose. Some pregnant women occasionally leak small quantities of urine during the second half of pregnancy and this is easily confused with amniotic fluid. Bath water does not normally enter the vagina, but sometimes during pregnancy a small quantity of water may do so and escape shortly after bathing. If a small rupture has occurred in the membranes it will result in a small but continuous leakage of amniotic fluid from the vagina. If she has any doubt about the origin of the water the patient should put on a sanitary towel, go to bed and, if the leakage continues, notify her doctor or telephone the hospital. If the leakage of amniotic fluid has occurred prematurely she will be admitted to hospital where it may be possible to prevent the onset of premature labour by rest and the administration of drugs.
Amount of Amniotic Fluid
The quantity of amniotic fluid varies during a pregnancy, as stated above, but it also varies from person to person and from pregnancy to pregnancy. Within reasonable limits the amount has no effect upon the foetus or upon its growth and development. In some pregnancies there is a small amount of amniotic fluid, but this does not mean that they are abnormal or that labour will be difficult. It does mean, however, that the baby has less room in which to turn and it is important that he should assume his correct position—cephalic (head) presentation—at about the 30th or 32nd week of pregnancy.
If an excessive quantity of amniotic fluid is present it is called polyhydramnios.