Development of Amniotic Fluid
From the very beginning of the formation of the extracoelomal cavity, amniotic fluid 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). 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.
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