BACKGROUND:An important contributor to fetal growth is growth of the placenta, the fetus' sole source of nutrients and oxygen. Here we use placental growth measures (larger and smaller disk diameters, reflecting the laterally expanding chorionic plate, and disk thickness) to test the hypothesis that placental growth patterns, while associated with placental weight and birth weight, measure placental functional efficiency, and will have independent effects on the feto-placental weight ratio (FPR).METHODS:Placental measures were available from 23,313 participants in the Collaborative Perinatal Project delivered between 34 and 43 completed weeks. Continuous variables were analyzed by regression for associations with placental weight, birth weight, and FPR, to further explore effects of placental growth patterns on the FPR (lateral chorionic plate growth and chorionic disk thickness were grouped as low, normal, and high values). The relationships of the nine resultant combinations of placental growth categories to the FPR using birth weight adjusted for gestational age, infant gender, parity, and African American race were analyzed (ANOVA).RESULTS:As chorionic disk area and thickness increased, birth weight and placental weight increased, and the FPR decreased (each p <.0001) after adjustment for gestational age, parity, race, and infant gender. Small, thin placental disks had an adjusted FPR of 8.46; the largest, thickest placentas had an adjusted FPR of 6.33. The nine categories of FPRs were significantly different, consistent with chorionic plate area and disk thickness combining to determine the FPR.CONCLUSIONS:Patterns of placental growth, relating to different functional dimensions of the placenta, deliver a different birth weight for a given placental weight.