During pregnancy, the fetus develops in the cavity and bathes in amniotic fluid. Composed of 96% water, this constantly changing liquid contains electrolytes, mineral elements (sodium, potassium, calcium, trace elements, etc.), amino acids, but also fetal cells.

The first traces of amniotic fluid appear soon after fertilization with the formation of the amniotic cavity on the 7th day. During the first weeks of pregnancy, the fluid is then essentially secreted by the embryo itself by a phenomenon of extracellular expansion (called extravasation). A minimal part of the fluid is also secreted by the mother via movements of water from the chorionic villi present in the future placenta. However, between 20 and 25 weeks, the skin of the fetus becomes impermeable (keratinization process). Therefore, the volume of amniotic fluid is guaranteed by a balance between what is excreted by the fetus (production) and what it swallows in utero.

• Fluid excretion is done mainly through two means:

  – Le syfetal urinary stema and more particularly the diuresis which is set up around 12-13 WA. After 20 weeks, it becomes the main source of production of amniotic fluid reaching 800 to 1200 ml / 24 hours at the end of pregnancy (against 110 ml / kg / d to 190 ml / kg / d at 25 weeks).

  – the lung fluid, secreted from 18 weeks, reaches 200 to 300 ml / 24h at the end of pregnancy.

• The reabsorption phenomenon amniotic fluid is possible thanks to the swallowing of the future baby. Indeed, the fetus swallows a large part of the amniotic fluid, which thus passes through its digestive system and its respiratory system, before being transmitted to the maternal organism and being, at the end of the race, filtered by the kidneys of the future mother.

Thanks to this “chain” of physiological production, the amniotic fluid follows a very particular cycle over the weeks of pregnancy to adapt to the weight and development of the future baby:

• Before 20 WA, the amount of amniotic fluid in the cavity gradually increases (from 20 ml at 7 WA to 200 ml at 16 WA),
• Between 20 weeks and 33-34 weeks, the volume stagnates around 980 ml,
• After 34 weeks, the volume of amniotic fluid decreases, with an acceleration of the phenomenon towards 39 weeks, the volume of fluid reaching approximately 800 ml at term.

  Variable according to the women, the volume of amniotic fluid is between 250 ml (low limit) and 2 liters (high limit), so that the pregnancy is said to be normal.

Amniotic fluid plays a variety of roles that change during pregnancy. The first and best known of its functions: protect the unborn child from shocks and noise.

But amniotic fluid also helps to:

• guarantee the stability of the environment of the fetus, maintaining a constant temperature and adapting its volume to the development of the baby,
• capture differences in taste, light, smell or hearing, thus promoting the child’s in utero sensory development.
• facilitate the movements of the fetus and participate in its good muscular and morphological development,
• provide the water and mineral salts that the future baby needs.
• lubricate, when the membranes rupture, the genital tract and thus prepare the body for the passage of the child.

But amniotic fluid is also a valuable indicator of fetal health. As such, the test to assess the amount of amniotic fluid is ultrasound. This can be recommended if the practitioner suspects an abnormality in the height of the uterus, a decrease in fetal movements or a premature rupture of the membranes. The sonographer may then have to use different techniques to assess a possible oligoamnios (decrease in the amount of amniotic fluid) or hydramnios (excess amniotic fluid, see below), namely:

The measurement of the largest vertical tank (CGV)

Also called Chamberlain’s method, the examination involves the ultrasound exploration of the entire amniotic cavity in order to locate the largest reservoir of fluid (the place where there is no interference with a member of the fetus or the umbilical cord). The measurement of its depth then guides the diagnosis:

• if it is less than 3 cm, the examination suggests oligoamnios,
• if it measures between 3 and 8 cm, it is normal,
• if it is greater than 8 cm, it may indicate hydramnios.

The amniotic index (ILA) measurement

This examination consists of dividing the umbilicus into 4 quadrants, then measuring and adding the depth of the tanks thus identified.

• if it is less than 50 mm, the risk of oligoamnios is high,
• if it measures between 50 mm and 180 mm; the amount of amniotic fluid is normal,
• if it is greater than 180 mm, a hydramnios should be considered.

Beyond the volume of the amniotic fluid, the practitioner may have to analyze the elements that compose it, as is the case when performing a amniocentesis. The objective: to look for an infectious agent if the context is in favor of a fetal infection or to study the chromosomes of the fetus to detect possible pathologies of genetic origin (starting with trisomy 21). In fact, the amniotic fluid contains numerous fetal cells in suspension, the concentration of which reaches its peak between 16 and 20 weeks. The cultivation of these cells makes it possible to produce a karyotype and thus to accurately assess certain risks of chromosomal abnormalities.

During the prenatal follow-up, the practitioner pays particular attention to the volume of amniotic fluid by measuring the uterine height. The objective: to exclude or take care of an insufficient (oligoamnios) or excessive (hydramnios) quantity of amniotic fluid, the 2 pathologies which can have potentially serious consequences on the consequences of pregnancy.

L’oligoamnios

L’oligoamnios is the most common amniotic fluid abnormality (between 0,4 and 4% of pregnancies). This insufficiency of amniotic fluid (less than 250 ml) can appear at different times during pregnancy and cause more or less serious complications depending on the stage of development of the fetus. The most frequent risks:

• Pulmonary hypoplasia (stopping the development of the lungs) generating, at birth, respiratory failure,
• anomalies of the musculoskeletal system (Potter sequence), the unborn child not being able to move in utero.
• premature rupture of membranes complicated by maternal-fetal infection and therefore increased risk of premature delivery, induction of labor or birth by cesarean section.

Its origins: various fetal causes (renal or urinary system malformation, chromosomal anomaly), maternal (gestational diabetes, CMV infection, etc.) or a placental disorder (transfusion-transfusion syndrome, poor vascularization of the appendages, etc.). The management of oligoamnios then depends on its main causes.

L’hydramnios

L’hydramnios describes an excess of amniotic fluid exceeding 1 to 2 liters. This anomaly can take two forms:

• chronic slow onset hydramnios usually appears around the third trimester of pregnancy and is fairly well tolerated.
• the acute hydramnios, quick to install is mostly seen in the second trimester of pregnancy. It is accompanied by clinical symptoms that are often poorly tolerated: uterine pain, breathing difficulties, contractions, etc. Rare, it would occur in 1/1500 to 1/6000 pregnancies.

 This abnormality in the volume of amniotic fluid can again have different causes. When it is of maternal origin, hydramnios may be due to gestational diabetes, pre-eclampsia, infection (CMV, parvovirus B19, toxoplasmosis) or Rh incompatibility between mother and child. But hydramnios can also be explained by anemia or certain malformations of the central nervous or digestive systems of the fetus.

And like oligoamnios, hydramnios presents a certain number of risk of complications: premature delivery, premature rupture of membranes, presentation of the baby in breech, cord procidence, maternal side; certain malformations in children, which vary according to the severity of the pathology.

In view of the diversity of the causes and the risks for the mother and the child, the care is assessed on a case-by-case basis.

• When it comes from a curable condition in utero or after birth (anemia, etc.), hydramnios is the subject of a specific treatment for the said pathology.
• Symptomatic management may also be recommended in certain cases. The practitioner then opts for a medical treatment based on anti-prostaglandins to reduce fetal diuresis or evacuating punctures to limit the risks of premature birth.
• In the most serious cases (anamnios), a medical termination of pregnancy can be considered after discussion with the parents.

Amniotic fluid is contained by two membranes, the amnion and the chorion, that make up the uterine cavity. When they rupture, they can cause fluid to flow. We then speak of rupture of the membranes or more commonly of rupture of the water bag.

• The rupture of the membranes at term is a sign of impending childbirth. Only antibiotic treatment to protect the child from possible infection can be recommended if labor does not begin within 12 hours of rupture, and induction planned within 24 to 48 hours in the absence of labor contractions.
• The rupture of membranes occurring before term is said to be premature. The objective of the management is then simple: delay as much as possible the premature delivery to reach ideally 37 WA. The follow-up then involves hospitalization until childbirth in order to facilitate regular assessments (infectious assessment, ultrasound, cardiac monitoring), antibiotic therapy to prevent a possible fetal infection, as well as corticosteroid-based treatment to accelerate the lung development (before 30 WA) of the unborn child. Note, however: a rupture of the membranes before 22 weeks often puts the vital prognosis of the fetus at stake.