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The amniotic fluid is the protective liquid contained by the
amniotic sac The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until sho ...
of a
gravid In biology and human medicine, gravidity and parity are the number of times a woman is or has been pregnant (gravidity) and carried the pregnancies to a viable gestational age (parity). These terms are usually coupled, sometimes with additional t ...
amniote Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are distin ...
. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products between mother and fetus. For humans, the amniotic fluid is commonly called water or waters (Latin liquor amnii).


Development

Amniotic fluid is present from the formation of the
gestational sac The gestational sac is the large cavity of fluid surrounding the embryo. During early embryogenesis it consists of the extraembryonic coelom, also called the chorionic cavity. The gestational sac is normally contained within the uterus. It is t ...
. Amniotic fluid is in the
amniotic sac The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until sho ...
. It is generated from maternal plasma, and passes through the fetal membranes by osmotic and hydrostatic forces. When fetal kidneys begin to function around week 16, fetal urine also contributes to the fluid. In earlier times, it was believed that the amniotic fluid was composed entirely of fetal urine. The fluid is absorbed through the fetal tissue and skin. After 22 to 25 week of pregnancy, keratinization of an embryo's skin occurs. When this process completes around the 25th week, the fluid is primarily absorbed by the fetal gut for the remainder of gestation.


Contents

At first, amniotic fluid is mainly water with electrolytes, but by about the 12-14th week the liquid also contains proteins, carbohydrates, lipids and
phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...
s, and
urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...
, all of which aid in the growth of the fetus.


Volume

The volume of amniotic fluid changes with the growth of fetus. From the 10th to the 20th week it increases from 25ml to 400ml approximately. Approximately in the 10th-11th week, the breathing and swallowing of the fetus slightly decrease the amount of fluid. Neither urination nor swallowing contributes significantly to fluid quantity changes until the 25th week when keratinization of skin is complete; then the relationship between fluid and fetal growth stops. It reaches a plateau of 800ml by the 28-week gestational age. The amount of fluid declines to roughly 400ml at 42 weeks. Some sources indicate about 500ml to 1000ml of amniotic fluid is present at birth.


Rupture of membranes

The forewaters are released when the amnion ruptures. This is commonly known as the time when a woman's "water breaks". When this occurs during labour at term, it is known as "spontaneous rupture of membranes". If the rupture precedes labour at term, however, it is referred to as "pre-labour rupture of membranes". Spontaneous rupture of membranes before term is referred to as "premature rupture of membranes". The majority of the hindwaters remain inside the womb until the baby is born. Artificial rupture of membrane (ARM), a manual rupture of the amniotic sac, can also be performed to release the fluid if the amnion has not spontaneously ruptured.


Function

Swallowed amniotic fluid (in later stages of development) creates urine and contributes to the formation of
meconium Meconium is the earliest stool of a mammalian infant resulting from defecation. Unlike later feces, meconium is composed of materials ingested during the time the infant spends in the uterus: intestinal epithelial cells, lanugo, mucus, amniotic ...
. Amniotic fluid protects the developing baby by cushioning against blows to the mother's abdomen, allowing for easier fetal movement and promoting muscular/skeletal development. Amniotic fluid swallowed by the fetus helps in the formation of the gastrointestinal tract. It also protects the fetus from mechanical jerks and shocks. The fetus, which develops within a fluid-filled amniotic sac, relies on the placenta for respiratory gas exchange rather than the lungs. While not involved in fetal oxygenation, fetal breathing movements (FBM) nevertheless have an important role in lung growth and in development of respiratory muscles and neural regulation. FBM are regulated differently in many respects than postnatal respiration, which results from the unique intrauterine environment. At birth, the transition to continuous postnatal respiration involves a fall in temperature, gaseous distention of the lungs, activation of the Hering-Breuer reflex, and functional connectivity of afferent O2
chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecept ...
activity with respiratory motoneurons and arousal centers.


Clinical significance


Collection

Amniotic fluid is removed from the mother by an
amniocentesis Amniocentesis is a medical procedure used primarily in the prenatal diagnosis of genetic conditions. It has other uses such as in the assessment of infection and fetal lung maturity. Prenatal diagnostic testing, which includes amniocentesis, is ne ...
procedure, where a long needle is inserted through the abdomen into the amniotic sac, using ultrasound guidance such that the fetus is not harmed. Amniocentesis is a low risk procedure, with risk of pregnancy loss between 1 in 1500 – 1 in 700 procedures. Amniocentesis can be performed to obtain diagnostic genetic information, evaluate for intrauterine infection, or rarely, to assess for fetal lung maturity if early delivery is required. If warranted, fluid is collected between 16 and 42 weeks of fetal development. The amount of fluid removed depends on the indication for the procedure and the testing that will be performed on the fluid.


Analysis

Analysis of amniotic fluid can reveal many aspects of the baby's genetic health as well as the age and viability of the fetus. This is because the fluid contains metabolic wastes and compounds used in assessing fetal age and lung maturity, but amniotic fluid also contains fetal cells, which can be examined for genetic defects. Amniotic fluid normally has a pH of 7.0 to 7.5.Vaginal pH Test
from ''Point of Care Testing'', July 2009, at: University of California, San Francisco – Department of Laboratory Medicine. Prepared by: Patricia Nassos, PhD, MT and Clayton Hooper, RN.
Because pH in the upper vagina is normally acidic (pH 3.8-4.5), a vaginal pH test showing a pH of more than 4.5 strengthens a suspicion of rupture of membranes in case of clear vaginal discharge in pregnancy. Other tests for detecting amniotic fluid mainly include nitrazine paper test and fern test. One main test that is performed on amniotic fluid is the L/S ratio test (lecithin/sphingomyelin). This test is used to determine fetal lung maturity. Both lecithin and sphingomyelin are lung surfactants that are present in increasing amounts in the maturing fetus, though past week 33, sphingomyelin levels remain relatively constant. Measuring a ratio of L/S of 2:1 or greater indicates that the fetus can be safely delivered, with functioning lungs.


Complications related to amniotic fluid

Too little amniotic fluid is called oligohydramnios. In a minority of cases it can be a cause of problems for the mother and baby. These include contracture of the limbs, clubbing of the feet and hands, and also a life-threatening condition called hypoplastic lungs. The Potter sequence refers to a constellation of findings related to insufficient amniotic fluid. On every prenatal visit, the obstetrician, gynaecologist or midwife should measure the patient's
fundal height Fundal height, or McDonald's rule, is a measure of the size of the uterus used to assess fetal growth and development during pregnancy. It is measured from the top of the mother's uterus to the top of the mother's pubic symphysis. Fundal height, ...
with a tape measure. It is important that the fundal height be measured and properly recorded to track proper fetal growth and the increasing development of amniotic fluid. The obstetrician, gynaecologist or midwife should also routinely ultrasound the patient—this procedure will also give an indication of proper fetal growth and amniotic fluid development. Oligohydramnios can be caused by infection, kidney dysfunction or malformation (since much of the late amniotic fluid volume is urine), procedures such as
chorionic villus sampling Chorionic villus sampling (CVS), sometimes called "chorionic ''villous'' sampling" (as "villous" is the adjectival form of the word "villus"), is a form of prenatal diagnosis done to determine chromosomal or genetic disorders in the fetus. It ent ...
(CVS), and preterm premature rupture of membranes (PPROM). Oligohydramnios can sometimes be treated with
bed rest Bed rest, also referred to as the rest-cure, is a medical treatment in which a person lies in bed for most of the time to try to cure an illness. Bed rest refers to voluntarily lying in bed as a treatment and not being confined to bed because of ...
, oral and intravenous
hydration Hydration may refer to: * Hydrate, a substance that contains water * Hydration enthalpy, energy released through hydrating a substance * Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound * ...
,
antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention ...
s,
steroid A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and ...
s, and amnioinfusion. The opposite of oligohydramnios is
polyhydramnios Polyhydramnios is a medical condition describing an excess of amniotic fluid in the amniotic sac. It is seen in about 1% of pregnancies. It is typically diagnosed when the amniotic fluid index (AFI) is greater than 24 cm. There are two clini ...
, an excess volume of amniotic fluid in the amniotic sac.
Amniotic fluid embolism An amniotic fluid embolism (AFE) is a very uncommon childbirth ( obstetric) emergency in which amniotic fluid enters the blood stream of the mother, triggering a serious reaction, which results in cardiorespiratory (heart and lung) collapse and ...
is a rare but very often fatal condition for both mother and child.


Medical applications

It is being used in some surgeries of the outside of the eye. It is also being studied for some orthopaedic conditions.


Stem cell research

Recent studies show that amniotic fluid contains a considerable quantity of stem cells. These
amniotic stem cell Amniotic stem cells are the mixture of stem cells that can be obtained from the amniotic fluid as well as the amniotic membrane. They can develop into various tissue types including skin, cartilage, cardiac tissue, nerves, muscle, and bone. The cell ...
s are pluripotent and able to differentiate into various tissues, which may be useful for future human application. Some researchers have found that amniotic fluid is also a plentiful source of non-embryonic stem cells. These cells have demonstrated the ability to differentiate into a number of different cell-types, including brain, liver and bone. It is possible to conserve the stem cells extracted from amniotic fluid in private stem cells banks.


See also

*
Liquid breathing Liquid breathing is a form of respiration in which a normally air-breathing organism breathes an oxygen-rich liquid (such as a perfluorocarbon), rather than breathing air. By selecting a liquid that is capable of holding large amounts of ...
* Potter's Syndrome * Twin-twin transfusion syndrome


References

{{DEFAULTSORT:Amniotic Fluid Body fluids Embryology Obstetrics Midwifery