Longitudinal assessment of amniotic fluid volume in monoamniotic twin gestations

The aim of this study is to describe normal amniotic fluid volume through gestation in a cohort of normal monoamniotic (MA) twins.

Our ultrasound database was queried for MA twin gestations from 2004 to 2011. Monochorionic twin pregnancies mimicking MA gestations, such as twin-twin transfusion syndrome, were excluded. Complicated MA gestations and higher-order multifetal gestations involving an MA pair were excluded. Thirty subjects were followed with serial amniotic fluid index (AFI) measurements from 15 to 32 weeks gestation. Using each AFI measurement as a unique data point, a quadratic regression model and a multi-level growth model were developed against gestational age (GA), providing a predicted AFI at each completed week, with a 95% confidence interval.

The quadratic regression least squares and multi-level growth models yielded the same curve comparing the AFI to the GA. Figure 1 depicts the model with the 95% confidence interval for normal amniotic fluid volume by GA for normal MA twins. Table 2 shows the 5th, 50th, and 95th percentiles for AFI by week from 15 to 32 weeks.

We have generated normative data for amniotic fluid volume across gestation in uncomplicated MA twins. This can be used as a reference when managing MA pregnancies.