Too Much or Too Little Amniotic Fluid

Amniotic fluid is a clear, yellow liquid that surrounds the fetus during its development inside the uterus. The fetus and amniotic fluid are contained within a sac called the "amniotic sac," which is made of two membranes: an inner membrane called the "amnion" and an outer membrane called the "chorion." Amniotic fluid begins to form as early as 12 days after fertilization, within the amniotic sac that surrounds the fetus throughout its development.

In the early weeks of pregnancy, amniotic fluid consists mainly of water. As the pregnancy progresses, the volume of amniotic fluid gradually increases, and its composition changes to include proteins, fats, hormones, antibodies, electrolytes, skin cells, fetal urine, and more.

The volume of amniotic fluid peaks around week 32 of pregnancy - averaging one liter. This volume remains stable until week 38 and begins to decrease thereafter.

A normal amount of amniotic fluid in the amniotic sac throughout pregnancy reflects the health of both the fetus and the mother.

Normal Volume of Amniotic Fluid

The amount of amniotic fluid varies from woman to woman, with the greatest variation occurring between weeks 32-33. The normal amount in this week range varies between 400-2,100 milliliters (5th-95th percentile).

Amniotic Fluid Production

In the first trimester of pregnancy, amniotic fluid consists mainly of water, originating from filtered maternal blood (maternal plasma). Towards the end of the first trimester, the fetus's kidneys begin to produce urine. As pregnancy progresses, the amount of urine produced by the fetus increases - until in the second half of pregnancy, the main source of amniotic fluid is fetal urine.

Therefore, a functioning and healthy urinary system in the fetus is essential for the normal production of amniotic fluid.
In addition, a smaller portion of the amniotic fluid is produced in the fetal lungs and secreted into the amniotic sac.

Amniotic Fluid Absorption

Most of the amniotic fluid absorption occurs when the fetus swallows the fluid, and the rest is absorbed through the placenta and the membranes of the amniotic sac. For this reason, the absorption of amniotic fluid also depends on the existence of a healthy and functioning digestive system in the fetus.

Normally, the balance in amniotic fluid volume is maintained by swallowing the amniotic fluid on one hand, and fetal urination on the other.

Amniotic Fluid Test

Amniotic fluid can be sampled ​during an amniotic fluid test (Amniocentesis). The fluid sampled in the test contains fetal cells, which can be grown in a laboratory to perform genetic or other tests on the fetus.

Amniotic Fluid Volume Assessment

There is a technical difficulty in accurately measuring the amount of amniotic fluid, so it is an estimate only and not an exact measurement. In the past, abdominal palpation techniques were used to estimate polyhydramnios or oligohydramnios, but today, using an ultrasound machine, it is possible to estimate and identify (without measurement) if the amount of fluid is normal. The assessment of amniotic fluid volume is performed during routine prenatal care to ensure it is within the normal range. Additionally, the assessment is also performed as part of a biophysical profile test - a non-invasive test aimed at assessing the condition of the fetus in the womb and specifically the risk of intrauterine death.

If there is a suspicion of low or excessive amniotic fluid, a more accurate measurement will be performed, also using ultrasound.

Polyhydramnios (excessive amniotic fluid) is detected in 1%–2% of all pregnancies. This condition can occur in a mild, moderate, or severe degree.

Information you should know about polyhydramnios

• Although polyhydramnios can be significant, in many cases, it may only be temporary.
• A mild case of polyhydramnios does not necessarily predict problems in the fetus.
• The earlier the case occurs in pregnancy or the greater the degree of polyhydramnios, the higher the chance of fetal morbidity.

The Causes

In most cases of mild polyhydramnios, the cause is unknown and the condition resolves on its own. Sometimes this condition occurs because the baby is larger than average or because it is a multi-fetal pregnancy. In cases of moderate to severe polyhydramnios, possible causes include:

• Unbalanced maternal diabetes: Can cause an increase in fetal urine and polyhydramnios.
• Fetal malformations: For example, a blockage in the digestive system resulting from the lack of development of the esophagus (esophageal atresia) can impair the absorption of amniotic fluid and lead to polyhydramnios. Malformations in the fetal nervous system can also cause this.
• Genetic factors or syndromes: For example, neurological syndromes in the fetus manifested by skeletal muscle defects can cause muscle atrophy, and consequently, impair the fetus's ability to swallow.
• Infections during pregnancy: For example, Parvovirus.

The Complications

Polyhydramnios can have implications for pregnancy and birth and may cause the following phenomena:

• Early contractions.
• Preterm birth.
• Unstable fetal presentation (may flip to breech presentation).
• Umbilical cord prolapse - in this situation, the umbilical cord may precede the head.
• Placental abruption - can be caused during the rupture of membranes (water breaking) as a result of a rapid drop in intrauterine pressure.
• Increased postpartum bleeding due to the uterus's difficulty in contracting.

It is important to note that these complications are usually associated with extreme polyhydramnios and are taken into account when a woman arrives at the delivery room. The delivery room staff knows how to take preventive measures when necessary and according to the level of excess fluid to reduce the chances of birth complications.

Oligohydramnios (low amniotic fluid) is detected in 1%-3% of all pregnancies. Among women with an increased risk of pregnancy complications, the prevalence is higher, reaching 19%-20%. This condition can occur in a mild, moderate, or severe degree.

The more severe the lack of amniotic fluid, or if the condition is detected in the first half of pregnancy, the higher the risk of fetal morbidity.

The Causes

In most cases, the cause of mild oligohydramnios is unknown, and it usually does not involve morbidity or complications. Possible causes for oligohydramnios include:

• Rupture of the amniotic membranes, also known as "water breaking": When the amniotic membranes open before the expected date of delivery, this can cause low amniotic fluid.
• Structural malformations: Causes disturbances in the development of the fetal urinary system, such as urinary tract obstructions or bilateral renal agenesis (failure of the kidneys to form).
• Genetic disorders: Such as kidney diseases.
• Placental insufficiency: A condition where, due to various reasons, the placenta does not provide enough oxygen and essential nutrients to the fetus. In these cases, a slowdown in fetal growth will usually also occur.
• Post-term pregnancies: After week 38, the amount of amniotic fluid begins to decrease. A prolonged pregnancy beyond the due date can naturally lead to a low amount of amniotic fluid.
• Maternal morbidity: Such as hypertension, gestational diabetes, preeclampsia, or dehydration - which may impair blood flow in the placenta (perfusion) and consequently result in low amniotic fluid.

Complications

Since amniotic fluid plays an important role in the development of fetal bones and muscles and in preventing pressure on the umbilical cord, low fluid levels can cause structural deformities, impaired lung development, and even fetal distress or intrauterine fetal death.

To summarize, it is important to emphasize once more that although conditions of excessive or insufficient amniotic fluid are not common, however, early detection, maintaining consistent prenatal care and regular monitoring of fetal movements can help prevent complications during pregnancy and childbirth.