Throughout pregnancy, baby bathes there, protected, warm, fed. It is essential for the development of the fetus and indicates the child is ready to be born. But not only ! Zoom in on the amniotic fluid.
What is amniotic fluid?
Amniotic fluid is an aqueous substance, composed essentially of 96% water, clear and sterile in which the fetus will bathe during pregnancy. It also contains desquamated epithelial cells in suspension, from the skin of the fetus and pulmonary secretions, as well as electrolytes, lipids, proteins, glucose, amino acids, enzymes, and finally, depending on the stage of the disease. pregnancy and baby development, hormones.
Where is he from ?
Amniotic fluid appears in the first week after fertilization, with the formation of the amniotic cavity. This will gradually grow and fill with a substance produced mainly by the embryo itself, then by the fetus, and is constantly renewed.
This production goes through two major phases. The first, a phenomenon called transudation, lasts until about the fourth month (between 20 and 25 weeks of amenorrhea). The amniotic fluid is then mainly secreted through the skin of the fetus and the amniotic membranes. Another phenomenon, called fetal diuresis, begins at 12 weeks. The fluid is produced through the urine of the fetus. Finally, from 18 weeks, it is also produced through pulmonary secretions, during the respiratory movements of the fetus.
When the baby begins his keratinization, that is to say his skin becomes impermeable and therefore no longer allows water to pass through, the second stage begins. It is his kidneys, sufficiently developed and able to function, which will ensure the main production of the liquid. The baby swallows the liquid around it (reabsorption), between 200 and 500 ml per day, and they eliminate it by turning it into urine. Through this process, the amniotic fluid is renewed every 3 hours, constantly. Reabsorption by swallowing also allows some of the fluid to pass through the intestine and then into the blood. It is then transmitted to the mother's body and eliminated by the mother's kidneys.
In total, there are eight sources of liquid production. In addition to those previously mentioned, there are sources considered to be minor: oronasal secretions, the transmembrane pathway through the amnion and transcordonal and transcutaneous transfers.
What is the normal volume of amniotic fluid?
The amount of amniotic fluid will vary throughout pregnancy, adapting to the size and needs of the fetus.
Between fertilization and 20 weeks production increases significantly, as does the weight of the fetus. Around 22 weeks production reached a peak speed with a volume of 60ml / day. After this phase, the volume produced remains constant until weeks 33-34 when it reaches its maximum: approximately 1L. The volume then begins to decrease gradually then drastically passed 39 weeks. The speed of production becomes negative.
The amniotic fluid cycle can be summarized in 5 main stages, depending on the term of pregnancy:
7 SA: 20ml
16 SA: 200ml
34 SA: 980ml
40 SA: 800ml
42 SA (term expired): 540 ml
The volume of amniotic fluid varies from woman to woman with a low limit of 250 ml and an upper limit of 2L. Pregnancy is considered normal when the amount of fluid is within this range. This is measured during the ultrasound. Below or above these references, it may indicate a problem and a risk to the health of the baby. Further tests will better identify the cause of the abnormality.
What is its role during pregnancy?
The role of amniotic fluid is essential but also multiple.
- From the start of pregnancy, it prevents the embryo from adhering to the amnion and surrounds it completely from the 4th month.
- It protects the fetus from external shocks but also from noise.
- It also protects it from infections thanks to its antibacterial properties and the presence, in particular, of antibodies.
- It enlarges the uterine cavity so that the fetus can actively move and thus develop its muscular system and ensure the growth of its skeleton.
- It guarantees a stable environment for the fetus, adapting its volume to its development and maintaining a constant temperature of 37.5 ° C.
- It provides the mineral salts and water that the fetus needs for development.
- It participates in its sensory development by capturing the differences in taste, smell, hearing and brightness.
- With the loss of water, it lubricates the genital tract and prepares them for childbirth.
Amniocentesis and other tests related to amniotic fluid
Amniotic fluid, its evolution, volume, color and composition, are linked to the health of the baby. Because it contains fetal cells, it can thus be taken in order to establish a karyotype which will be studied to detect possible genetic pathologies, such as trisomy 21 and certain risks of chromosomal abnormalities. This examination, called amniocentesis, can be performed from the 14th week. The amniotic fluid is withdrawn with a needle that the doctor pricks through the abdomen of the pregnant woman, under local anesthesia. The cells which are extracted from it are then cultured and the results are obtained after 3 to 4 weeks.
If the ultrasound suggests an abnormality in the volume of amniotic fluid (insufficient or excess), the practitioner may recommend a measurement of the largest vertical cistern (CGV) or a measurement of the amniotic index (ILA). The first method is to find, using an ultrasound of the amniotic cavity, the place containing the most fluid and to evaluate the depth. It must measure between 3 and 8 cm to be normal. Below it can be a sign of oligoamnios and above it indicates hydramnios. The second method is also done by ultrasound and consists of calculating the amniotic index by adding the largest vertical cisterns of fluid in the 4 quadrants of the uterus obtained by virtual division by two perpendiculars crossing at the level of the umbilicus. The index has 5 levels. From 0 to 5 cm, it translates an oligoamnios, from 5.1 to 8 cm, it indicates a marginal level; from 8.1 to 18 cm, the volume is normal; between 18.1 and 25 cm, the liquid is in excess; greater than 25 cm, it indicates a hydramnios.
Finally, when the term is exceeded and the pregnancy is placed under surveillance, the health of the fetus can be checked using an amnioscopy which allows the color of the amniotic fluid to be visualized. The procedure is painless and is done by inserting a small tube into the opening of the cervix that the doctor puts in contact with the amniotic membranes. Then he illuminates them and observes by transparency the color of the amniotic fluid. It must indeed be clear, very slightly bluish. Staining may indicate the presence of more or less stool (meconium), or even blood, and may indicate fetal distress.
Oligoamnios, when the amniotic fluid is insufficient
If one of the different tests reveals an insufficient amount of amniotic fluid during pregnancy, it is called oligoamnios. This is the most common anomaly, affecting between 0.4 and 4% of pregnant women. In very rare cases the fluid may be completely absent and this is referred to as anamnios.
Oligoamnios does not necessarily manifest itself early in pregnancy but can appear at different times with more or less serious consequences on the health of the fetus depending on its stage of development. This insufficiency can cause:
- Pulmonary hypoplasia, that is, a stop in the development of the lungs, and therefore respiratory failure at birth
- Facial dysmorphia (Potter syndrome)
- An abnormality in the development of the motor system, or even a malformation of the limbs, the fetus not having been able to move in the cavity
- A maternal-fetal infection, the fetus no longer being sufficiently protected
- The death of the fetus
The causes of oligoamnion can be various, but the most common is premature rupture of membranes. It can also be caused by a fetal malformation in the renal and urinary tract, in particular, high blood pressure of the mother, gestational diabetes, intrauterine growth retardation, a placental disorder or even exceeding the term of pregnancy.
Its management and treatment depend on the cause and stage of pregnancy.
Hydramnios, when the amniotic fluid is in excess
We speak of hydramnios when the volume of amniotic fluid exceeds 2 liters. This anomaly can also occur at any time during pregnancy, quite suddenly. However, there are two types: chronic hydramnios, which is fairly common and which generally appears around the third trimester and at the end of pregnancy, and acute hydramnios, which is rarer (it affects between 1 pregnant woman in 1,500 and 1 in 6,000 ) and which can be observed most often in the second trimester.
The risks are various:
- Uterine contractions, premature rupture of membranes and therefore premature birth
- Poor positioning of the child at the time of childbirth (breech presentation)
- Breathing difficulties or edema in the mother
Here again, the causes may be maternal (gestational diabetes, toxoplasmosis, pre-eclampsia, blood incompatibility between mother and child) or fetal (anemia, malformation of the digestive system or the central nervous system). The care and treatment will not be the same depending on the cause.
Rupture of a water bag: loss of amniotic fluid
When the amnion and the chorium, the two membranes containing the amniotic fluid, rupture, they release it and we talk about the rupture of the water bag, or the loss of water. When it comes near term, as early as 37 weeks, this rupture means the baby is ready to go out and delivery is imminent. Labor usually begins within hours but it does not, and no contraction occurs, it can be induced. Antibiotic treatment may also be prescribed because in the absence of this protective bag, the child is exposed to infections.
If the rupture occurs prematurely, it is premature and may therefore lead to a premature birth. Before 22 weeks, the vital prognosis of the fetus is at stake. The expectant mother is usually hospitalized and every effort is made to delay delivery until, ideally, 37 weeks. Checkups are numerous and regular, and treatment may include antibiotics to prevent infection of the exposed baby as well as corticosteroids to speed lung development and prevent respiratory failure.
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