Amniotic Fluid and the Biophysical Profile
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)
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 "Biophysical Profile" to assess
whether a baby's in danger or not (See below). A normal AFI is about
12. But 8-18 is normal, too. If there's ample amount of fluid,
then we can only assume the baby's kidneys are functioning normally, and
then indirectly assume the kidneys are being perfused normally, driven
by a normal fetal blood pressure, and normal heart activity--in other words,
everything's working fine right on down the line.
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.
