Mean surface shape of a human placenta

Goal: The chorionic plate (or “fetal surface”) of the human placenta is drawn as round, with the umbilical cord inserted roughly at the center, but variability of this shape is common. The average shape of the chorionic plate has never been established. The goal of this work is to measure the average shape in a birth cohort. Materials and Methods: 1227 placentas collected from a prospective cohort had digital photographs taken of the chorionic plate after trimming of the membranes from the margin. A trained observer marked the perimeter of the fetal surface at intervals of between 1cm and 2cm. 1024 of the placentas were delivered at term. Of these, 44 cases were excluded because the measurement of chorionic plate landmarks was not possible. This left N=980 cases for analysis, 95.7% of all placentas delivered at term. Mathematical methods to estimate the average chorionic plate shape included calculating the median shape, and the mean perimeter. Mean values of the fetal surface area and the radial distance from the edge to the umbilical insertion point were also calculated. Results: The average chorionic plate is round and centered about the umbilical cord insertion. The median shape is round, with the average radius from the umbilical insertion point equal to R=9.066 cm (range 8.76 − 9.46 cm, mean square deviation 2%). The mean perimeter method gives R=9.01cm (with range 8.71 − 9.27 cm, mean square deviation 1.9%). The mean chorionic plate area is calculated to be equal to 285.58 cm, corresponding to R=9.53 cm. Conclusions: The average fetal surface shape for a placenta delivered at term is a disk around the umbilical insertion point. Different approaches to calculating the average agree on this and give similar estimates for the radius of the disk of about 9cm. Both the average radius from the umbilical insertion point, and the surface area of an individual placenta can serve as reliable measurements of the deviation from the mean shape.