Amniotic fluid volume increases with the duration of pregnancy, with about 200 cc at 16 weeks to about a liter between 28 and 36 weeks. Then it falls slightly with approaching term, to about 800 cc at 40 weeks. After 40 weeks, the volume drops further.
Amniotic fluid is removed by the fetal membranes, swallowed by the fetus, and in the presence of ruptured membranes, may leak out through the vagina. It is deposited in the amniotic sac by the fetal membranes and by fetal urination. Any disturbance in the normal equilibrium of fetal swallowing, urinating, or amniotic membrane fluid transport can result in oligohydramnios.
Oligohydramnios is both a symptom and a threat. As a symptom, it can reflect decreased (or absent) fetal renal output, congenital anomaly, or abnormal membrane fluid transport. Regardless of it's cause, oligohydramnios presents a threat to the fetus because the umbilical cord may be compressed more easily, resulting in impaired blood flow to the fetus.
Several means of identifying oligohydramnios are used, and they are not in complete agreement. The concept of oligohydramnios is universally accepted. The specific definition of oligohydramnios is not. Definitions have included:
When present in a woman not in labor, consideration is given to inducing labor early, depending on the clinical situation. During labor, oligohydramnios is sometimes treated with amnioinfusion, a deposit of sterile fluid into the amniotic sac to expand the AF volume. This is most frequently done to relieve fetal heart rate decelerations thought to be due to umbilical cord compresssion, or to try to clear some thick meconium that may be present.
OB-GYN 101: Introductory
Obstetrics & Gynecology
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