Fetal liver mass vs. gestational age.
The cubic growth model (solid line) given by Eq 31 was selected as the most parsimonious model in our analysis; however, that model gives negative liver mass values during early gestation. The Gompertz model (dashed line) given by Eq 32 is strictly positive on the time domain of interest. The model of Luecke et al. , which is a power law model based on fetal mass, was calibrated using a different data set . The model of Zhang et al.  was also calibrated using a distinct compiled data set. The Zhang et al.  model describes liver volume, so the units were converted assuming a tissue density of 1 g/mL.