Amniotic fluid is fascinating. Afterall, before our birth, we float in amniotic fluid, which provides temperature stability, cushioning, and a necessary presence within collapsed airways to help stimulate lung development.
By the beginning of the second trimester there are 50 cc of fluid in the amniotic sac, and this fluid isn’t much different from the baby’s plasma, indicating an origin from secretions through the umbilical cord, membrane coverings of the placenta, and even the baby’s skin. By the 36th week there is usually around a liter of amniotic fluid, but by this time it is made up for the most part from fetal urine. The turnover of fluid is fairly rapid, with a build up from urine and a reabsorption from fetal swallowing being important dynamics in the amniotic fluid picture from hour to hour. Since the baby’s kidneys mature over the gestation, the amniotic fluid is more fetal urine-like later than it is when the kidneys are less mature.
Also included in the amniotic fluid are the old skin cells of the baby which have nowhere to go but into this bath. (In the outside world we shed skin cells all day long; they fly off into the air and that’s that.) Chemicals from the lungs (often detectable as signs of lung maturity) are present later in pregnancy. Also, sodium, potassium, and other electrolytes are present as part of the exchange between those two dynamic entities, mother and child.
The amniotic fluid is crucial to lung development. When there’s little fluid, like in congenital abnormality of the bladder or missing kidneys, the trachea and other respiratory structures don’t mature, indicating that the pressure and nature of the fluid bathing these structures is important in their growth. This is a significant risk of premature rupture of membranes. Health care providers made great strides in preventing preterm labor and infection with premature rupture of membranes in the second trimester, only to have lung immaturity haunt us later.
So you can see that amniotic fluid isn’t just made of one particular substance, but of many. And depending where you are in your pregnancy, the make up of this fluid can be different.