Oligohydramnios
means too little amniotic fluid.
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:
- Visibly reduced AFV on ultrasound
- No vertical pocket of AF >2 cm
- No two-dimensional pocket of AF > 2 x 2 cm
- Amniotic fluid index (AFI) of <7 (or <6, or <5). AFI is the sum of the
single deepest pocket of amniotic fluid in each of the 4 quadrants, in cm.
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.
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