Amniotic Fluid and the Biophysical Profile

December 14, 2007

The "load" of pregnancy isn't just the baby. There's considerable weight and volume contributed by the placenta and the amniotic fluid. The amount of amniotic fluid has a certain range of normal values, the numbered score derived by adding up the centimeters of depth of four pockets of fluid seen on ultrasound. Some researchers feel one good pocket of 3 centimeters depth is enough to assume that there is adequate amniotic fluid around the rest of the baby. But the Fluid Index(AFI) is traditionally the addition of the four pockets, with the normal range from 8 to about 18. There are fuzzy limits beyond these two extremes, but generally...

AFI < 5-6 = oligohydramnios (oligo, less; hydramnios, fluid)

AFI > 18-22 = hydramnios or polyhydramnios (poly, much or many)

pregnant woman

Most of the fluid in amniotic fluid is contributed to by fetal urine. This is then resorbed by the membranes and umbilical cord and the turnover is pretty fast--a couple of hours.  So it's possible to have differing amounts of amniotic fluid from one day to the next, even from one hour to the next. The Amniotic Fluid Index (AFI) can be used to determine fetal well-being. It is part of the more complete

On the other hand, a low AFI (oligohydramnios) at or near term may be an indication for delivery, either by induction or C-section. If there were to turn out to be a normal amount of fluid with rupture of membranes during an induction, then the low AFI that prompted the induction was either temporary or wrong.It happens, but ultrasound's the best thing we have to go by, even with its inherent error.

The Biophysical Profile

  • AFI, as described above.
  • Fetal Movement
  • Fetal Tone
  • Fetal Breathing Movements, another sensitive indicator of fetal well-being. The fetus doesn't really "breathe," in the sense that he or she exchanges air, but the chest walls move, going through the motions.
  • Fetal Activity Determination (FAD), also referred to as a non-stress test (NST), in which accelerations of fetal heart rate are compared in real-time to fetal movement.

Each parameter can have a score of 0, 1, or 2.  A perfect score would be 10 out of 10.  A score of 6 or above can probably be closely observed. 5 or less probably would indicate delivery.  8 or greater is probably a stable pregnancy.

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