This is an excerpt from book. See end for source.
Amniotic fluid is the ocean-like substance unborn babies float in. It offers fetuses buoyancy, protection from trauma, and oxygen. Like semen, amniotic fluid is comprised of two basic elements: living cells and the liquid they’re suspended in. In this case, the cells represent sloughed-off fetal skin and bladder tissue.
Amniotic fluid is a mixed drink, with contributions from both baby and mother. Some proportion of amniotic fluid is secreted by the lining of the amniotic sac itself, and some of it is blood serum from the mother, which passes freely through this lining. And some of it is baby pee. We know this because fetuses without kidneys (who can survive only until birth) are surrounded by too little amniotic fluid. But what goes around comes around. Fetal urine itself is distilled from amniotic fluid, which is continuously sipped and swallowed by the baby. Amniotic fluid also soaks right through the skin because the outer waterproofing layer doesn’t form until week 20. The fetus also inhales it during rehearsals for breathing. In these ways, amniotic fluid bathes the inside as well as the outside of the developing body.
Amniotic fluid is a biological mystery. It is bacteriostatic, meaning that bacteria will not grow when cultured in it, so amniotic fluid undoubtedly helps keep the womb a sterile place. But what it does once it seeps inside of the fetus—through the mouth, through the lungs, through the skin—is not at all clear. Some researchers suspect it plays an integral role in establishing the fetal immune system. As it washes through, amniotic fluid exposes the respiratory and gastrointestinal tracts to various immunological factors. This contact may recruit the mucus lining of these sites for future work in immunity.
Whatever its internal activity, amniotic fluid eventually re-enters the womb as urine. Here it is absorbed back into the body of the mother and replaced by fresh fluid, in a ceaseless cycle of emptying and refilling. This process speeds up as pregnancy progresses. By the third trimester, amniotic fluid will turn over every three hours. By birth, every hour. But at fifteen weeks, the baby and I require twenty-four hours to replace the volume of fluid just removed.
The obstetrician is finishing up. She reminds me to drink plenty of water today.
Drink plenty of water. Before it is baby pee, amniotic fluid is water. I drink water, and it becomes blood plasma, which suffuses through the amniotic sac and surrounds the baby—who also drinks it.
And what is it before that? Before it is drinking water, amniotic fluid is the creeks and rivers that fill reservoirs. It is the underground water that fills wells. And before it is creeks and rivers and groundwater, amniotic fluid is rain. When I hold in my hands a tube of my own amniotic fluid, I am holding a tube full of raindrops. Amniotic fluid is also the juice of oranges that I had for breakfast, and the milk that I poured over my cereal, and the honey I stirred into my tea. It is inside the green cells of spinach leaves and the damp flesh of apples. It is the yolk of an egg. When I look at amniotic fluid, I am looking at rain falling on orange groves. I am looking at melon fields, potatoes in wet earth, frost on pasture grasses. The blood of cows and chickens is in this tube. The nectar gathered by bees and hummingbirds is in this tube. Whatever is inside hummingbird eggs is also inside my womb. Whatever is in the world’s water is here in my hands.