Even before the fetus is developed, a fluid-filled space develops around the embryo. The space is lined with a smooth and slippery, glistening membrane and filled with liquid. This clear, odorless, sterile liquid surrounds and cushions the baby. It is completely replaced every few hours. At full term, there is between 500-1000 cc of amniotic fluid. The amniotic fluid contains substances such as albumin, urea, uric acid, creatinine, lecithin, sphingomyelin, bilirubin, fat, fructose, leukocytes, proteins, epithelial cells, enzymes, vernix and lanugo. The presence of meconium staining indicates fetal defecation, which is the way the baby responds to stress such as a reduced oxygen supply.

The amniotic fluid has a number of important functions: it keeps the fetus at an even temperature, cushions against possible injury and provides a medium in which it can move easily. The baby will swallow the fluid, urinate into it, and be kept warm and protected by it. If the baby's presenting part is not closely applied to the area around the cervix, the hydrostatic action of the amniotic fluid works to dilate the cervical canal. The amount of fluid is controlled by a complex system of fluid exchange between mother and baby.

Amnionic fluid index (AFI) involves dividing the maternal abdomen into four quadrants using the umbilicus and linea nigra as the horizontal and vertical reference points. Holding an ultrasound transducer perpendicular to the floor, the vertical diameter of the largest pocket of amniotic fluid is identified and measured. The numbers from each quadrant are added together and the sum is the AFI. Decreased amniotic fluid is defined by an AFI less than or equal to 5 cm and increased amniotic fluid is an AFI greater than or equal to 24 cm.

Hydramnios, sometimes also known as polyhydramnios, is an excessive amount of amniotic fluid. As previously mentioned, the volume of amniotic fluid increases during pregnancy to approximately 1 liter or 1000 cc. A diagnosis of hydramnios is made when there is more than 2000 cc of amniotic fluid present. In rare cases, the uterus may contain enormous quantities of amniotic fluid (as much as 15 liters). The major source of amniotic fluid is the amnionic epithelium layer. However, in hydramnios, no significant changes in these cells would lead to a diagnosis of too high production of fluid. Researchers hypothesize that the baby's inability to drink amniotic fluid; fluid trapped in the fetal body; conditions such as anencephalus, spina bifida, and enlarged placenta; and twinning may contribute significantly to hydramnios. Approximately 45% of abnormalities are associated with the central nervous system, 30% include gastrointestinal anomalies, and 7% include cardiovascular problems. Some women experience maternal symptoms such as, pain and difficulty breathing, and need treatment. Bed rest may help, and diuretics and restrictions of water and salt are ineffective. Even though some women may not discover this condition until late in their pregnancy (if ultrasound has not been performed); others find that labor may begin spontaneously at about the 28th week with premature rupture of membranes (possibly followed by cord prolapse) or perinatal death.

Oligohydramnios is the term used when the amniotic fluid level falls below the normal limits. Often oligohydramnios is an indication of renal anomalies such as: incomplete kidney(s), the absence of the kidneys, or an obstruction of the urethra. If the fetus is unable to urinate the amniotic fluid, it results in a decreased amount. Some mothers with oligohydramnios may be overdue and often run a higher risk of cord compression due to decreased ability of the baby to float. Variable decelerations are seen with oligohydramnios and an amnioinfusion, the infusion of normal saline through an intrauterine pressure catheter (IUPC) can be used to decrease the variable decelerations and decrease the risk of cord compression as the labor continues.

References:

Lowdermilk, D., and Perry, S. Maternity & Women's Health Care. Mosby/Elsevier: St. Louis, Missouri. 9th Edition, 2007.

Reeder, S., Martin, L., and Koniak-Griffin, D. Maternity Nursing: Family, Newborn and Women's Health Care. Lippincott: Philadelphia & New York. 18th Edition, 1997.

Pritchard, J., MacDonald, P., and Gant, N. Williams Obstetrics. Appleton-Century-Crofts: Norwalk, Connecticut, 17th Edition, 1985.