AMNIOTIC FLUID VOLUME IN TWIN PREGNANCIES

Most reports (1-3) on amniotic fluid volume in pregnancy describe a total amniotic fluid volume rather than an index that is specific to each fetus of a twin pregnancy. This concept results in the inability to make judgments about the amniotic fluid of each twin separately.

A recent report (4) describes an index that is specific to each fetus. The difficulty that arises is the inability to accurately delineate the entire separating membrane, especially in advanced gestations.

• Technique (Gerson and co-workers) (4):
• Each fetal sac is divided into two portions using the fetal diaphragm as the dividing line.
• The largest fluid pocket (devoid of cord) above and below the diaphragm is measured.
• Therefore two measurements are derived for each fetus. These two values are added to provide an amniotic fluid index (AFI) for each fetus (A and B).
• This results in a twin specific AFI.
• The estimate for the larger and smaller AFI were used to develop one sided 95% confidence limits along with the appropriate averages.
• Each fetus's AFI is reported to "occupy a place in its own curve that is related to the place of the AFI of its sibling" (4).
• A trend in successive AFI indices may be a clue to twin-twin transfusion syndrome.

REFERENCES

1. Watson W, Harlass F, Menard M et.al. Sonographic assessment of amniotic fluid volume in normal twin pregnancy. Am J Perinatol 1995;12:122-124.
2. Chau AC, Kjos SL, Kovacs BW. Ultrasonographic measurement of amniotic fluid volume in normal diamniotic twin pregnancies. Am J Obstet Gynecol 1996;174:1003-1007.
3. Porter TF, Dildy GA, Blanchard JR et.al. Normal values for amniotic fluid index during uncomplicated twin pregnancy. Obstet Gynecol 1996;87:699-702.
4. Gerson A, Free SM, Russino J et.al. Amniotic fluid index in twin gestation. Ultrasound Obstet Gynecol 1997;10:98-102.