Waterworld: Amniotic Fluid
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. We've 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.
Since most of the amniotic fluid comes from the baby's urine, any congenital urinary problem--absence of kidneys or blockage of urination--will lead to a dangerously decreased amniotic fluid volume. And just as decreased urine in these conditions lead to abnormally low amounts of amniotic fluid, so also does any impairment in fetal swallowing cause an abnormal increase in the fluid. Most of the re-uptake of amniotic fluid in the turnover is accomplished by the baby swallowing. Esophageal atresia, in which the esophagus doesn't develop or when there's no connection between the esophagus and the stomach, leads to such dangerously high amounts of fluid that premature labor can occur. In these situations, periodic draining by needle (amniocentesis) can be beneficial; but of course the diagnosis of esphogeal atresia must be addressed.
Less urine output = less fluid; less swallowing = more fluid.)
So the whole birth experience is reminiscent of the transition from aquatic to land-dwelling in our species. In any normal pregnancy, the amniotic fluid is an important part of the package deal.
It's amazing we ever learn to stop wetting the bed.