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Amniotic fluid or liquor amnii is the protective liquid contained by the amniotic sac of a pregnant female.
From the very beginning of formation of the extracoelomal cavity, amniotic fluid [AF] can be detected. This firstly water-like fluid originates from the maternal plasma, and passes through the fetal membranes by osmotic and hydrostatic forces. As the placental and fetal vessels develop, the fluid passes through the fetal tissue, as the exsudatum of the skin. After the 20th-25th week of pregnancy when the keratinization of skin occurs, the quantity of amniotic fluid begins to depend on the factors that comprise the circulation of AF.
At first it is mainly water with electrolytes, but by about the 12-14th week the liquid also contains proteins, carbohydrates, lipids and phospholipids, and urea, all of which aid in the growth of the fetus.
The volume of amniotic fluid is positively correlated with the growth of fetus. From the 10th to the 20th week it increases from 25ml to 400ml approximately. From the 8th week, when the fetal kidneys begin to function, fetal urine is also present in the AF. Approximately in the 10th week the breathing and swallowing of the fetus slightly decrease the amount of AF, but neither urination nor swallowing contributes significantly to AF quantity changes, until the 25 week, when keratinization of skin is complete. Then the relationship between AF and fetal growth stops. It reaches the plateau of 800ml at the 28 week gestational age (GA). The amount of fluid declines to roughly 400 ml at 42 weeks GA. 
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" (SROM). If the rupture precedes labour at term, however, it is referred to as "premature rupture of membranes" (PROM). 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.
Amniotic fluid is "inhaled" and "exhaled" by the fetus. It is essential that fluid be breathed into the lungs in order for them to develop normally. Swallowed amniotic fluid also creates urine and contributes to the formation of meconium. 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.
Analysis of amniotic fluid, drawn out of the mother's abdomen in an amniocentesis procedure, can reveal many aspects of the baby's genetic health. This is because the fluid also contains fetal cells, which can be examined for genetic defects.
Amniotic fluid normally has a pH of 7.0 to 7.5. 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.
Recent studies show that amniotic fluid contains a considerable quantity of stem cells. These amniotic stem cells 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. Some private companies offer this service for a fee.
Too little amniotic fluid (oligohydramnios) can be a cause or an indicator of problems for the mother and baby. The majority of pregnancies proceed normally and the baby is born healthy, but this isn't always the case. Babies with too little amniotic fluid can develop contractures of the limbs, clubbing of the feet and hands, and also develop a life threatening condition called hypoplastic lungs. If a baby is born with hypoplastic lungs, which are small underdeveloped lungs, this condition is potentially fatal and the baby can die shortly after birth due to inadequate oxygenation. Potter sequence refers to a constellation of findings related to insufficient amniotic fluid and includes shortened and malformed limbs with clubbed feet and the underdeveloped lungs that can lead to perinatal death.
On every prenatal visit, the obstetrician/gynaecologist or midwife should measure the patient's 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 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 (CVS), and preterm premature rupture of membranes (PPROM). Oligohydramnios can sometimes be treated with bed rest, oral and intravenous hydration, antibiotics, steroids, and amnioinfusion. It is also important to keep the baby warm and moist.
The opposite of oligohydramnios is polyhydramnios, an excess volume of amniotic fluid in the amniotic sac.