What is Amniotic Fluid?【Supervised by doctor】

What is Amniotic Fluid?

Amniotic fluid is the liquid that surrounds the fetus in the uterus during pregnancy. It plays a role in reducing pressure and shock on the fetus and maintaining a favorable environment within the uterus.

Abnormalities in the amount of amniotic fluid can have a significant impact on the fetus, making it crucial to monitor during pregnancy. Additionally, amniocentesis is performed as a prenatal diagnostic test to examine the fetus for chromosomal abnormalities and congenital anomalies.

Color and Odor of Amniotic Fluid

In the early stages of pregnancy, amniotic fluid is colorless and transparent, but in the later stages, it becomes milky white as fetal skin cells and vernix mix in. Normally, it has no odor. The pH of amniotic fluid is alkaline, ranging from 7.0 to 8.5.

Amniotic fluid can become cloudy, which may occur due to infection or when the fetus passes meconium (the baby’s first stool) into the amniotic fluid.

How is the Amount of Amniotic Fluid Measured? What is the Normal Amount?

The amount of amniotic fluid gradually increases from the early stages of pregnancy, peaking at about 800 ml between 30 and 35 weeks, and then decreases until delivery.

The measurement of amniotic fluid is done via ultrasound (echo examination), which is a regular part of prenatal checkups. While ultrasound cannot directly measure the volume of amniotic fluid, it evaluates it using metrics such as the Amniotic Fluid Index (AFI) and the Maximum Vertical Pocket (MVP).

The Amniotic Fluid Index is calculated by dividing the uterus into four quadrants centered around the navel, measuring the depth of the amniotic fluid in each quadrant, and summing these four measurements (the normal range is 5-24 cm). The Maximum Vertical Pocket is a simpler measurement, determining the largest distance between the uterine wall and the fetus (the normal range is 2-8 cm).

Components of Amniotic Fluid

Amniotic fluid contains various substances such as electrolytes, glucose, amino acids, and lipids. These substances primarily come from the blood of the mother and fetus, as well as the fetus’s excretions (urine) and lung secretions.

From the mid-pregnancy period onwards, the fetus swallows amniotic fluid, which then enters the respiratory tract and lungs, gets absorbed in the intestines, and is excreted as urine. This urine is then swallowed again, creating a cycle of circulation of the amniotic fluid.

If amniotic fluid, which contains fetal cells and tissues, enters the mother’s bloodstream for any reason, it can cause amniotic fluid embolism, leading to severe symptoms such as respiratory distress and low blood pressure.

Functions of Amniotic Fluid

Amniotic fluid has various roles, including cushioning the fetus from external shocks, creating a space within the uterus that allows the fetus to move easily, and helping to maintain a consistent temperature for the fetus. These functions are crucial for the physical development and growth of the fetus. Additionally, the growth factors and bioactive substances present in amniotic fluid are known to promote the development of the gastrointestinal tract, respiratory system, and other organs.

Beyond the development and growth of the fetus, amniotic fluid also plays an important role during labor. It helps to distribute the pressure from uterine contractions and assists in dilating the cervix through the amniotic sac (amniotic sac), facilitating the birthing process.

Abnormalities of Amniotic Fluid

As previously explained, amniotic fluid is crucial for the growth and development of the fetus, as well as for labor. However, an abnormal amount of amniotic fluid, whether too much or too little, can indicate significant abnormalities occurring in the fetus.

Polyhydramnios

An excessive amount of amniotic fluid is called polyhydramnios. It can be broadly categorized into two types: when the abnormality is on the fetal side and when the abnormality is on the maternal side.

When There Are Abnormalities on the Fetal Side

The increase in amniotic fluid due to fetal abnormalities can be further divided into two categories: increased production of amniotic fluid and impaired absorption of amniotic fluid.

Causes of increased production of amniotic fluid from the fetus include conditions like blood type incompatibility pregnancies, which can lead to fetal anemia and increased cardiac output.

Impaired absorption of amniotic fluid can be caused by chromosomal abnormalities or central nervous system abnormalities that weaken the swallowing ability, or by gastrointestinal abnormalities such as esophageal atresia or duodenal atresia. It is known that polyhydramnios due to duodenal atresia has a high likelihood of being associated with Down syndrome.

When There Are Abnormalities on the Maternal Side

If the mother has diabetes, the fetus can also have high blood sugar levels, leading to an increase in the amount of fetal urine, which in turn can cause an increase in amniotic fluid.

Oligohydramnios

A decrease in amniotic fluid can occur when the production of fetal urine decreases, or if there are congenital abnormalities in the kidneys or obstructions/narrowing in the urinary tract (ureters or urethra).

Conditions such as pregnancy-induced hypertension or fetal growth restriction can also lead to a poor environment for the fetus, resulting in reduced urine production and subsequently less amniotic fluid. Additionally, the loss of amniotic fluid due to premature rupture of membranes (PROM) can also cause a decrease in amniotic fluid.

Amniotic Fluid Cloudiness

Cloudiness of amniotic fluid mainly occurs when the fetus passes meconium (the baby’s first stool) inside the uterus. It becomes more likely as the pregnancy progresses, with reports indicating that it reaches 30% after 42 weeks of gestation. Meconium-stained amniotic fluid reflects fetal maturity and does not in itself require special treatment. However, if the fetus inhales the cloudy amniotic fluid, it can lead to “meconium aspiration syndrome,” which can cause damage to the lungs and airways, leading to respiratory distress. Therefore, caution is necessary.

What is Amniocentesis?

Amniocentesis is a type of prenatal test performed before the baby is born. It involves collecting amniotic fluid and analyzing the fetal cells within it to check for chromosomal or genetic abnormalities in the fetus.

The test is conducted between 15 and 18 weeks of pregnancy. Using an ultrasound to determine the position of the fetus and placenta, a thin needle is inserted through the abdomen to collect the amniotic fluid (the procedure is performed under local anesthesia). The collected cells are then cultured and analyzed for chromosomal abnormalities, which can take several weeks to obtain the final results.

Amniocentesis is indicated when either parent is a carrier of chromosomal abnormalities, if there has been a previous child born with chromosomal abnormalities such as Down syndrome, if the mother is of advanced maternal age, or if the NIPT (Non-Invasive Prenatal Testing) result is positive.

Although it is not considered a highly dangerous procedure, amniocentesis involves inserting a needle into the uterus, which carries a risk of miscarriage of approximately 0.1-0.3%.

Currently, it is considered difficult to use amniocentesis to diagnose developmental disorders such as autism.

NIPT (Non-Invasive Prenatal Testing)

NIPT (Non‐Invasive Prenatal Testing) is a type of prenatal testing that analyzes fragments of fetal DNA present in the mother’s blood to check for chromosomal abnormalities such as Down Syndrome. Unlike amniocentesis, it does not involve sampling amniotic fluid.

Since it can be done with a blood test, it is simpler compared to amniocentesis. However, because it is a non-diagnostic test, if a positive result is obtained from NIPT (Non‐Invasive Prenatal Testing), a diagnostic test such as amniocentesis is necessary to confirm the diagnosis.

Article Editorial Supervisor

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Dr Hiroshi Oka

NIPT specialist clinic, MD

Graduated from Keio University, School of Medicine