The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. 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. Amniotic fluid is in the amniotic sac. 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 phospholipids, and urea, 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 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 by the 28-week gestational age. The amount of fluid declines to roughly at 42 weeks. Some sources indicate about 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