AFI =Amniotic Fluid Volume

Determination  of amniotic Fluid  volume

As   originally  described by Phelan   and associates  in order to calculate the  AFI   the abdomen is arbitrarily  divided  into four quadrants   in which the  umbilicus divides the upper  and    lower  halves  and the linea  nigra   divides   the right  and left halves. With the patient   supine the linear  transducer  is placed along the maternal anterior   abdominal  wall and   held perpendicular  to the floor. The maximum  vertical  pocket  of fluid   ( usually  reported  in centimeters )   equals the sum of  largest single vertical pocket of fluid  that is  at least  1 cm  in width  obtained following  the same  criteria described here .

AFV  has been   shown to result in over diagnosis of low AFV  ( oligohydramnios ).  In a  study    by Magann and colleagues     the use of color   Doppler   inappropriately  diagnosed  21% of  women with   low AFV  who actually had normal   dye determined AFVs . In  addition  color Doppler  did not identify  any more  pregnancies  with true dye determined low AFVs compared  with traditional   gray  scale ultrasound imaging .

Oligohydramnios  (Low AFV   referred to as oligohydramnios )  has been defined  as any one of the following .1)  A total  volume less than 200 mL  or less  than 500 mL  a value  below the 5 th percentile  for gestational   age 2)  an SDP ( single deep pocket ) less than 2 cm 3) an AFI    less than  5cm,  or 4) a subjectively low AFV.

An increased  AFV ( referred to as polyhydramnios ) can be defined    as any  one of the  following : A) a total volume  greater  than 2000 mL a value above  the 95^th  or 97  percentiles for gestational age B)  an SDP (single deep pocket ) greater  than   or equal  to 8 cm  C) an  AFI  greater  than or equal to 24 cm or greater  than 25 cm or D) a subjectively increased  AFV,

Polyhydramnios :-The   incidence  of polyhydramnios  ( also  referred to as hydramnios ) ranges  from 0.2%  to 2.0%   . The degree of polyhydramnios  can be described using  the terms mild moderate and  severe . Mild  polyhydramnios has been   defined as an a)  AFI  of 25 to 30 cm  or b)  a DVP  of 8 cm   or greater. But   moderate   polyhydramnios as A)  an  AFI   of 30. 1 cm  to 35 cm or B) a DVP of   12 cm or greater  . However,   severe polyhydramnios  as an 1)  AFI  of 35.1  cm or greater  2) or a DVP of   16 cm  or greater.

Idiopathic polyhydramnios accounts for approximately 50%    to 60%  of cases. The remaining  cases typically fall into  one of the  following   categories :  congenital   anomalies and genetic     disorders ( 8-45% ) , maternal  diabetes (  5- 26% )  multiple  gestations ( 8-10% )  fetal anemia   (1-11%)  and other   ( e.g.  hydrops fetalis , Bartter syndrome and congenital  viral  infections )   The mechanism  by which   idiopathic polyhydramnios  develops is not known . Memembrane  bound  water channels called aquaporins may play   a role in the  development   of polyhydramnios    but the exact physiology  is not yet understood.

 Increasing   severity of polyhydramnios correlates with an increased risk  of perinatal  death and congenital abnormalities .. Up to  31%   of pregnancies with severe    polyhydramnios  (AFI >35 ) have   a major   congenital    anomaly. The most   common structural anomalies associated with polyhydramnios are central nervous system cardiac tor gastrointestinal malformations  . The  risk of fetal aneuploidy in these   fetuses found to have an anomaly by sonography is 10%  . In those fetuses without sonographic evidence  of an anomaly the risk of aneuploidy  is only 1%   . The most   common trisomy 21,  trisomy 18,    and trisomy 13,   although other chromosomal  abnormalities can also  occur . There is not a significant  difference in the reported risk  of fetal  aneuploidy with increasing severity of polyhydramnios.

The  proposed  mechanism of polyhydramnios associated with maternal diabetes  is related  to fetal  polyuria   due to increased osmotic dieresis as a result of fetal  hyperglycemia . Poorer   glucose control  has been  shown to correlated with higher  amniotic  glucose concentration and higher  AFI. There may   also be an increase  in fetal  urinary   output in macrocosmic  fetuses ( which   are often   seen in diabetic  pregnancies )  The incidence  of polyhydramnios   in mothers   with  progestational   diabetes  after   24 weeks gestational  age has been shown  to be 18.8%    . In the setting of gestational diabetes the   incidence of polyhydramnios ranges from 8%  to 20%  and is found  up to 30 times  more often   than in non diabetic  pregnancies.