What is Amniotic Fluid?

Amniotic fluid (liquor amnii) is the clear or slightly yellowish liquid surrounding the fetus in the uterus. [1] The amniotic sac of a pregnant woman contains the amniotic fluid which plays a vital role in the proper development of the baby. [2]

Amniotic Fluid Composition

It mainly contains water, fetal wastes (mainly urine) and fetal skin cells. [3]

Amniotic Fluid Origin and Production

The fluid is produced by the mother’s placenta during the first trimester and the early part of the second trimester, until the baby’s kidneys are mature enough to take over the task. [3] The baby swallows the fluid as they “breathe” and then excretes it again as urine, thus maintaining the constant circulation of the fluid. However, the urine making up the fluid is not pure waste material as the majority of the fetal waste is passed through the placenta to be filtered by the mother’s kidneys.

What are the Functions of Amniotic Fluid?

It fulfils various purposes apart from protecting the baby inside the womb by forming a cushion around it.

  • Helps with the uniform growth of the body parts and organs of the baby [2]
  • Assists with the proper bone and muscle development
  • Allows the baby to move inside the uterus
  • Prevents the amniotic sac wall from sticking to the baby
  • Breathing it in and out while in the uterus ensures proper lung growth of the baby
  • The swallowed fluid creates urine and helps with the production of meconium (earliest stools of a newborn infant)
  • Allows the digestive system of the baby to develop properly
  • Prevents the umbilical cord (responsible for carrying food and oxygen to the fetus) from being squeezed [3]
  • Maintaining a constant temperature to keep the baby healthy [4]

Amniotic Fluid Levels

Normal Amniotic Fluid Levels

amniotic fluid picture

Its volume continues to increase until the 34th to 36th week of pregnancy [5] when the amniotic sac contains around one quart fluid on average. The fluid volume then gradually goes down until the time of delivery. [6] Having too much or too little amniotic fluid during pregnancy may lead to various health conditions and complications related to childbirth.

The amniotic sac breaks during or before (rare cases) labor draining the fluid through the vagina. This is commonly referred to as the waters breaking.

Low Amniotic Fluid Levels

Amniotic fluid deficiency during pregnancy can lead to a Oligohydramnios, which increases the chances of complications like premature birth and various birth defects (like hypoplastic lungs) in the newborn as well as miscarriage and stillbirth. There are generally no symptoms of low fluid levels apart from the belly being smaller than it should at a certain gestational age. Risk factors include gestational diabetes, hypertension, high blood pressure and preeclampsia. The treatment mainly involves fetal monitoring using regular ultrasounds. [8]

High Amniotic Fluid Levels

Excessive amniotic fluid level in the uterus is known as polyhydramnios which can lead to symptoms like difficulty breathing, excessive weight gain and edema. Various factors may be responsible for elevated fluid levels, such as maternal diabetes and infectious conditions as well as fetal abnormalities. The extra amniotic fluid may leak through the vagina in some rare cases. Complications associated with polyhydramnios are similar to oligohydramnios as they include preterm labor, premature rupture of membranes, stillbirth and various congenital problems (cleft palate, Down’s syndrome). Treatment or monitoring often includes weekly ultrasounds and karyotyping. [9]

Amniotic Fluid Index Chart

Amniotic fluid index or AFI helps to estimate the amniotic fluid levels in the uterus for determining the fetal well-being. It is included in the biophysical profile of the fetus.

The AFI (usually expressed in cm) is determined by performing ultrasound (ultrasonography) examination of the uterus. Certain procedures are used by doctors for determining the AFI with the “single deepest pocket” and the four-quadrant technique being most commonly used. The latter technique involves measuring the deepest vertical length of the fluid pockets separately in each quadrant to calculate the total fluid volume.

The AFI is given below (in centimeters) by gestational age in a normal pregnancy [7]:

Week 2.5th 5th 50th 95th 97.5th
16 7.3 7.9 12.1 18.5 20.1
17 7.7 8.3 12.7 19.4 21.1
18 8.0 8.7 13.3 20.2 22.0
19 8.3 9.0 13.7 20.7 22.5
20 8.6 9.3 14.1 21.2 23.0
21 8.8 9.5 14.3 21.4 23.3
22 8.9 9.7 14.5 21.6 23.5
23 9.0 9.8 14.6 21.8 23.7
24 9.0 9.8 14.7 21.9 23.8
25 8.9 9.7 14.7 22.1 24.0
26 8.9 9.7 14.7 22.3 24.2
27 8.5 9.5 14.6 22.6 24.5
28 8.6 9.4 14.6 22.8 24.9
29 8.4 9.2 14.5 23.1 25.4
30 8.2 9.0 14.5 23.4 25.8
31 7.9 8.8 14.4 23.8 26.3
32 7.7 8.6 14.4 24.2 26.9
33 7.4 8.3 14.3 24.5 27.4
34 7.2 8.1 14.2 24.8 27.8
35 7.0 7.9 14.0 24.9 27.9
36 6.8 7.7 13.8 24.9 27.9
37 6.6 7.5 13.5 24.4 27.5
38 6.5 7.3 13.2 23.9 26.9
39 6.4 7.2 12.7 22.6 25.5
40 6.3 7.1 12.3 21.4 24.0
41 6.3 7.0 11.6 19.4 21.6
42 6.3 6.9 11.0 17.5 19.2


Complications Associated With Amniotic Fluid

Apart from the above two complications, amniotic fluid abnormalities can lead to the following conditions as well:

Amniotic Fluid Embolism (AFE)

A rare condition in which the amniotic fluid as well as some fetal materials such as hair, fetal cells and other debris enters the mother’s bloodstream through the placental bed, triggering allergic reactions. The principal symptoms are shortness of breath, sudden decrease in blood pressure, seizures, nausea, pulmonary edema and cardiovascular collapse. It can lead to life threatening complications such as severe neurological damage and even brain death. [10]

Amniotic Band Syndrome (ABS)

A group of birth defects generally occurring when certain body parts of the growing fetus gets caught in thin strings or bands within the womb. It occurs when the inside membrane of the placenta ruptures while the outer one remains intact, causing the stringy pieces to float around in the fluid. Sometimes, these pieces can entangle around the growing baby, cutting off the blood circulation in certain body parts (commonly the fingers and toes). Associated birth defects include short or absent finger or toe, cleft lip, cleft palate and club foot. Surgery in the utero to disentangle the strings is the most common and effective treatment option. [6]

Chorioamnionitis (Intra-Amniotic Infection)

Also known as amnionitis, it is a bacterial infection of the amniotic fluid before or during labor. E. coli, anaerobic bacteria and group B streptococci are some of the most common causes of the infection. Associated complications include heavy blood loss during and after delivery, c-section delivery and bacteremia (bacteria in the blood of the mother and the baby). [11]

Meconium Aspiration Syndrome

Sometimes, the baby passes their first feces (meconium) shortly before delivery which then mixes with the amniotic fluid. Meconium aspiration syndrome, occurring when the baby inhales this mixture into the lungs, can lead to various congenital conditions (chronic lung disease, hearing loss, limpness at birth) [16] and even death of the infant. [12]

Amniotic Fluid Tests and Analysis

Amniotic fluid analysis involves collecting a fluid sample from the mother’s abdomen (amniocentesis[13] to assess the genetic health of the developing baby. The fetal cells in the fluid help to determine the risk of any genetic defects. Amniocentesis test is also useful for gender determination.

Vaginal pH tests can also help to detect various fetal abnormalities. The amniotic fluid pH level ideally ranges between 7.0 and 7.5 while the upper vaginal pH remains between 3.8 and 4.5. So, a pH test strip showing pH levels above 4.5 may indicate ruptured membranes. Other tests used for detecting fetal abnormalities and fluid leakage include fern test and nitrazine paper test. Regular fundal height measurement can also help to ensure proper fetal growth and detect any change in the fluid levels.

Signs of Leaking Amniotic Fluid

Amniotic fluid leakage through the vagina can indicate certain serious complications during any pregnancy stage. A constant feeling of wetness due to continuous vaginal discharge is the main sign of leaking amniotic fluid. However, sometimes it might be quite difficult to determine whether the dampness is occurring due to fluid leakage or is simply caused by excessive sweating or urine leakage (mainly during the third trimester). The following points may help to detect a fluid leakage:

  • The smell of the vaginal discharge can help to determine if it is amniotic fluid as the fluid has a characteristic sweetish smell rather than the normal ammonia odor of urine.
  • The amniotic fluid may be cloudy or have a light yellowish or greenish or brownish (in case of meconium syndrome) coloration which can help with the identification. Sometimes, the discharge may have a pinkish or reddish tint due to blood in the amniotic fluid, which may indicate some fetal abnormality. However, some women may have clear amniotic fluid (which does not help with the identification of the discharge).

How to Increase Amniotic Fluid Levels?

Certain medical procedures are used for temporarily increasing the amniotic fluid levels for managing the conditions associated with low fluid amounts.

  • Amnioinfusion [14]: It allows increasing the quantity of amniotic fluid by instilling a saline solution into the amniotic sac.
  • Maternal Re-Hydration [15]: This procedure involves rehydrating the mother’s body using oral and IV fluids. Due to this reason, pregnant women with low amniotic fluid levels are often asked by their doctors to drink lots of water.

Amniotic Fluid and Stem Cells

Experts have recently proved that considerable amounts of stem cells are present in the amniotic fluid. These pluripotent cells are capable of differentiating into various tissues (skin, cartilage bones, muscles), a property that may prove useful for medical applications in the future. [17]

  1. http://www.nlm.nih.gov/medlineplus/ency/article/002220.htm
  2. http://www.marchofdimes.com/pregnancy/amniotic-fluid.aspx
  3. http://www.parentingweekly.com/pregnancy/pregnancy_information/amniotic_fluid.htm
  4. http://www.nhs.uk/chq/Pages/2310.aspx?CategoryID=54
  5. http://www.ivillage.co.uk/amniotic-fluid-infections/81857
  6. http://babyworld.co.uk/2011/10/amniotic-fluid/
  7. http://fetalanomalies.org/Fluid.html
  8. www.babycentre.co.uk/a568740/low-amniotic-fluid
  9. http://americanpregnancy.org/pregnancycomplications/highamnioticfluidpolyhydramnios.htm
  10. http://www.mayoclinic.com/health/amniotic-fluid-embolism/DS01207
  11. http://emedicine.medscape.com/article/973237-overview
  12. http://www.hopkinschildrens.org/meconium-aspiration-syndrome.aspx
  13. http://www.webmd.com/baby/guide/amniocentesis
  14. http://www.mayoclinic.com/health/low-amniotic-fluid/AN01659
  15. http://www.parentingweekly.com/pregnancy/pregnancy-complications/low-amniotic-fluid_2.htm
  16. http://kidshealth.org/parent/medical/lungs/meconium.html#a_Possible_Long_Term_Complications
  17. http://www.ncbi.nlm.nih.gov/pubmed/19281660 [/ref]