Amniotic fluid provides a supportive and protective environment for fetal development during pregnancy. A decreased amniotic fluid volume (oligohydramnios) can occur because of fetal anomalies, intrauterine growth restriction, pre-eclampsia or prolonged (post-term) pregnancy. Many caregivers practice planned delivery by induction of labor or caesarean section after diagnosis of decreased amniotic fluid volume at term, to prevent an adverse pregnancy outcome. Ultrasonography is non-invasive and is used widely for the follow up of pregnancy. It can be used to determine amniotic fluid volume by measuring either the amniotic fluid index or single deepest vertical pocket.
This review demonstrated that using the amniotic fluid index increased the number of pregnant women who were diagnosed with oligohydramnios and induced for an abnormal fluid volume when compared with the deepest vertical pocket measure. The women also had a higher rate of caesarean section for so-called fetal distress. Yet the rate of admission to neonatal intensive care units and the occurrence of neonatal acidosis, an objective assessment of fetal well-being, were similar between the two groups. The other measured perinatal outcomes that were no different were a non-reassuring fetal heart rate tracing, the presence of meconium, or an Apgar score of less than 7 at five minutes. These conclusions were from five randomized controlled trials involving 3226 women with singleton pregnancies, reported on between 1997 and 2004.
The accurate assessment of amniotic fluid volume by ultrasonography can be influenced by an inexperienced operator, fetal position, the probability of a transient change, and the different ultrasound diagnostic criteria of an abnormal volume.