Amniocentesis, polyhydramnios, oligohydramnios, chorioamnionitis –you may have heard of some, or all, of these terms in association with pregnancy. They all are connected, as they all include the root amnio. That’s the Greek word for bowel, it’s usage derived from the shape of the amniotic sac that contains the fetus. Composed of an outer membrane called the chorion and an inner membrane called the amnion, the amniotic sac also contains clear, pale to yellow fluid called amniotic fluid, which supports and protects the fetus for various reasons.
One function of the amniotic fluid is to lubricate the fetus and all of its anatomic parts, particularly the distal extremities (fingers and toes), which otherwise can become webbed (indeed, webbing is one possible result of oligohydramnios, a condition in which there is an inadequate volume of amniotic fluid). The amniotic fluid also supports and lubricates the umbilical cord, thereby preventing cord compression. Prevention of cord compression is vital as a vein in the cord (called the umbilical vein) delivers oxygen and nutrients from the placenta to the developing fetus, while a pair of arteries (the umbilical arteries) carries blood that is depleted in oxygen (O2) and rich in waste products (including carbon dioxide [CO2]) to the placenta. The placenta, in turn, is where exchange of O2, nutrients, and waste products takes place between the maternal and fetal blood. O2 can move from the material to fetal blood, because the hemoglobin inside fetal red blood cells (RBCs) has a higher affinity for O2 than adult hemoglobin has. Although the placenta keeps maternal blood separate from fetal blood, the blood vessels carrying maternal and fetal blood within the placenta come into close proximity. This allows O2 to move from maternal RBCs, where the O2 is held by hemoglobin similar to how a magnet holds iron filings, to fetal RBCs, where the fetal hemoglobin grabs the O2 similar to how a stronger grabs the iron filings. The proximity of the maternal and fetal vessels also allows the nutrients and waste products to move between the two blood supplies.
Amniotic fluid also contains antibodies that help protect the fetus against infection, plus the fluid provide buoyancy, so that the developing muscles can move the fetus and strengthen, and so the developing nervous system can learn to control the muscles. This furthermore enables the development of the fetal bones. While floating within the amniotic fluid, the fetus both swallows fluid and takes fluid into the lungs, which furthermore helps fetal development. The fetus also urinates into the amniotic fluid.
Over the course of pregnancy, the volume of amniotic fluid increases gradually to accommodate the fetus. Oligohydramnios, a condition in which there is not enough amniotic fluid for the gestational age, can result from a range of issues. It can develop due to inadequate blood supply (uteroplacental insufficiency), various fetal health problems such as restricted growth, various medications, and, most commonly, premature rupture of membranes (your “water breaks” too early). Early rupture of membranes also puts a mother and fetus at risk of the amnion and chorion becoming infected. Known as intraamniotic infection, or chorioamnionitis, this condition has a high rate of both maternal and neonatal complications and death, if not recognized quickly and treated with antibiotics. (Johnson et al., 2017) Rupture of membranes also tends to initiate labor, so it is not bad if it occurs when you reach term.
The opposite of oligohydramnios is polyhydramnios, a condition in which there is too much amniotic fluid for the gestational age. Occurring in 1-2 percent of pregnancies, this condition this condition usually is mild and mostly causes discomfort for the mother. However, if moderate or severe, polyhydramnios can lead to dyspnea (difficulty breathing) and swelling in the lower extremities. By increasing the elbow room for the fetus, polyhydramnios also can lead the fetus to flip around from the head-down position to a breech position, making it more likely that your obstetrician will opt for a cesarean delivery. Because it increases the pressure in the womb, polyhydramnios also can stimulate preterm delivery.
Because amniotic fluid contains, not only hormones, glucose, and various other substance, but also fetal cells, which in turn contain fetal DNA, the fluid is extremely useful for testing to evaluate the health of the fetus. For any of several reasons, your obstetrician may recommend amniocentesis, the drawing of amniotic fluid via needle for testing. These reasons include a need for genetic testing of the fetus, a need to test the fetal lungs, a need to assess an infection, an abnormal prenatal blood test, and abnormal ultrasound, or because the mother is 35 years old, or older. One other reason for amniocentesis is to release amniotic fluid in cases of severe polyamnios. Usually, amniocentesis is performed somewhere from the 15 to 20th week of pregnancy. Along with revealing any problems, genetic testing following amniocentesis will reveal the fetal gender.