The clinically important question with any test is does it
accurately discriminate between affected and healthy individuals so
that affected individuals can be treated without doing undure harm
to healthy ones? The answer with all forms of tests of fetal
wellbeing, including amniotic fluid volume measurements, is "no."
Here's what I found when doing the research for the induction
chapter for the forthcoming new edition of Obstetric Myths
Versus Research Realities:
- I could find only one study in 1600 women in which results of
amniotic fluid measurements were concealed. (Concealment is
important because knowing the results leads to inductions, and
inducing labor leads to more fetal distress and cesareans
independent of fetal condition.) Low amniotic fluid volume was
associated with increased likelihood of low blood pH at birth, but
the authors concluded that the sensitivity (test accurately
identifies affected individuals) was too low at 29% to make the
test useful for identifying at-risk fetuses. We also, of course,
don't know if even in compromised fetuses,
induction overstressed a fetus who would have been able to
tolerate a spontaneous labor.
Morris JM, Thompson K, Smithey J, et al. The usefulness of
ultrasound assessment of amniotic fluid in predicting adverse
outcome in prolonged pregnancy: a prospective blinded observational
study. BJOG 2003;110(11):989-94.
- We have a trial assigning 900 women to routine AFI
evaluation at hospital admission in early labor or not.
Investigators found that women having AFI doubled their odds of
cesarean for fetal distress (6.5% vs. 3.2%) with no difference in
Chauhan SP, Washburne JF, Magann EF, et al. A randomized study
to assess the efficacy of the amniotic fluid index as a fetal
admission test. Obstet Gynecol 1995;86(1):9-13.
- We have a systematic review of trials comparing two techniques
of amniotic fluid volume estimation. More women with AFI were
diagnosed with having low amniotic fluid volume than with
single deepest pocket (22% vs. 9%), which resulted in more
inductions (14% vs. 7.5%) but no differences in neonatal
Nabhan AF, Abdelmoula YA. Amniotic fluid index versus single
deepest vertical pocket as a screening test for preventing adverse
pregnancy outcome. Cochrane Database Syst Rev
- Finally, we have a study showing that low amniotic fluid volume
in the absence of other symptoms is not concerning. Investigators
compared 200 women induced for isolated low amniotic fluid volume
with 400 similar women with normal fluid volume and spontaneous
labor onset. Women induced for low fluid volume were more likely to
have cesareans (16% vs. 6%) and cesareans for nonreassuring fetal
status (8% vs. 2%) without improving neonatal outcomes.
Manzanares S, Carrillo MP, Gonzalez-Peran E, et al. Isolated
oligohydramnios in term pregnancy as an indication for induction of
labor. J Matern Fetal Neonatal Med 2007;20(3):221-4.
And, of course, we also know that amniotic fluid volume is not
fixed but varies according to maternal hydration.
Depressing, isn't it?