Amnionic fluid volume in prolonged pregnancy

The evidence implicating reduced amnionic fluid volume as a major problem in prolonged pregnancy seems clear. Oligohydramnios serves to explain the association of umbilical cord compression fetal heart rate patterns in labors complicated by fetal distress and such cord entrapment may even explain the release of meconium into the amnionic fluid. Ultrasonic assessment of amnionic fluid volume, regardless of the definition of oligohydramnios used, appears to single out the prolonged pregnancy where the fetus is at risk. Important questions remain as to how best to quantify fluid volume using sonography and, importantly, whether normal amnionic fluid volume reliably predicts fetal well being for a predictable period of time. That is, oligohydramnios diagnosed using ultrasound seems meaningful, but whether a normal fluid volume permits safe expectant management needs further evaluation. This is particularly relevant if the liquor volume may subside within 24 to 48 hours as reported by Beischer et al. Unfortunately, the mechanisms by which amnionic fluid volume decreases in prolonged pregnancies remain unknown. As in Ballantyne's time, we still do not know "... what functions the fetus is performing during these accessory weeks ... nor how he is performing them ..." But we have learned that oligohydramnios has a central role in producing some of the fetal complications associated with prolonged pregnancy.