Amniotic sac: Protected living space
The amniotic sac is a sac consisting of egg membranes that increasingly fills with fluid (amniotic fluid) as the child grows. This allows the growing child to swim freely, attached only to the umbilical cord. This enables the child to build up its muscles and skeleton and to grow evenly.
The amniotic fluid also performs a number of other tasks: It prevents the egg membranes and embryo from growing together, stimulates lung maturation and protects the unborn child from mechanical damage. External shocks, for example, are cushioned and the baby remains unharmed thanks to the full bladder. In addition, the umbilical cord and its vessels remain free to move and can supply the baby with the best possible care.
The valuable fluid also helps with thermoregulation: development and growth stimulate the baby’s metabolism enormously. This produces a lot of heat, which the unborn child can release via the amniotic fluid. This prevents temperature fluctuations, so overheating or hypothermia are not possible.
Shortly before birth, the filled amniotic sac also helps to open the cervix. Before or during birth, the amniotic sac bursts (rupture of the membranes), allowing the liquid contents to flow out.
Formation and composition of amniotic fluid
The amniotic fluid is provided by both the mother and the baby. Up to the twelfth week of pregnancy, it comes mainly from the mother, being released via the placenta. In the second half of pregnancy, the baby largely takes over the task of production.
Around the 14th week of pregnancy, the growing baby begins to drink amniotic fluid. This stimulates the gastrointestinal tract and the amniotic fluid is filtered by the kidneys. From time to time, the baby empties its bladder, which guarantees a constant exchange of amniotic fluid. However, the baby’s lungs, the membranes and the placenta also play a role in the exchange. Towards the end of the pregnancy, the amniotic fluid is completely replaced approximately every three hours.
Amniotic fluid volume
In the tenth week of pregnancy, the amniotic sac is filled with around 30 milliliters of amniotic fluid. In the 20th week of pregnancy it is already 350 to 500 milliliters. The maximum of 1,000 to 1,200, sometimes even 2,000 milliliters is reached in the 36th week of pregnancy. After that, the amount drops to 800 to 1,000 milliliters.
Too much amniotic fluid
In rare cases, the amount of fluid in the amniotic sac is too high. Doctors then speak of polyhydramnios. You can find out more about this in the article Too much amniotic fluid.
Too little amniotic fluid
Determining the amount of amniotic fluid
The attending doctor uses ultrasound to determine the amount of fluid in the amniotic sac. He must take into account that the amount varies from woman to woman even in normal cases and depends on the age of the growing child. There is therefore no absolute standard value. The quantity can be determined in a variety of ways:
Amniotic fluid index
One of the most common methods is to determine the so-called amniotic fluid index (FI). During the ultrasound examination, the abdomen is divided into four quadrants (areas) and the largest amniotic fluid deposit is determined in each. The sum of the four measurements gives the FI. In the last trimester of pregnancy, the value is normally between five and 20 centimeters. Values below five centimetres indicate too little amniotic fluid, values above 20 centimetres indicate too much.
Deepest amniotic fluid depot
Another option is to measure the so-called deepest amniotic fluid depot. Here, the doctor measures the vertical distance from one side of the membranes to the other. A length of around two to eight centimeters is considered normal. Values under two centimetres indicate too little amniotic fluid, values over eight centimetres indicate too much.
This is the most common measurement method for multiple pregnancies.
Two-diameter amniotic fluid depot
Experience of the doctor
The experience of the attending physician is not insignificant when determining the volume. His trained eye is usually sufficient to recognize deviating amounts of amniotic fluid. Additional results from the ultrasound measurement then enable him to make a reliable statement about the amount of fluid in the amniotic sac.
What does amniotic fluid look like?
The color of amniotic fluid is yellowish-clear in the 15th/16th week of pregnancy. Towards the due date, the color changes to whitish-cloudy.
Green amniotic fluid: transfer
A missed due date is often accompanied by a change in the color of the fluid: amniotic fluid can become cloudy due to the baby’s first stool excretions (meconium) and take on a green color. The doctor may then decide to induce labor. This is because if the amniotic fluid mixed with stool gets into the baby’s lungs (meconium aspiration), it can sometimes be life-threatening for the newborn. Therapeutic aspiration of the lungs is therefore an important first medical measure.
Amniocentesis during pregnancy
During the procedure, the doctor uses a fine cannula to puncture the abdominal wall and uterine wall of the pregnant woman and aspirate some of the amniotic fluid. This contains fetal cells that are examined in the laboratory for genetic defects. Other substances in the fluid also provide information about possible infections or fetal diseases such as an open back.
Amniocentesis is monitored by ultrasound for safety, is hardly painful and is usually completed after five to ten minutes. It is usually carried out between the 14th and 20th week of pregnancy.
Possible risks
Amniocentesis can result in contractions or slight bleeding. The risk of miscarriage is low at 0.5 to 1 percent. Nevertheless, women should take it easy for several days after amniocentesis.
