Probability distributions for measures of placental shape and morphology

Birthweight at delivery is a standard cumulative measure of placental growth, but is a crude summary of other placental characteristics, such as, e.g., the chorionic plate size, and the shape and position of the umbilical cord insertion. Distributions of such measures across a cohort reveal information about the developmental history of the chorionic plate which is unavailable from an analysis based solely on the mean and standard deviation. Various measures were determined from digitized images of chorionic plates obtained from the pregnancy, infection, and nutrition study, a prospective cohort study of preterm birth in central North Carolina between 2002 and 2004. Centroids (geometric centers) and umbilical cord insertions were taken directly from the images. Chorionic plate outlines were obtained from an interpolation based on a Fourier series, while eccentricity (of the best-fit ellipse), skewness, and kurtosis were determined from the method of moments. Histograms of each variable were compared against the normal, lognormal, and Lévy distributions. Only a single measure (eccentricity) followed a normal distribution. All others followed lognormal or 'heavy-tailed' distributions for moderate to extreme deviations from the mean, where the relative likelihood far exceeded those of a normal distribution.