Aherne''s observation (that the human placenta grows as the two power of the fetal weight) can be combined with previous equations on the growth of the fetus as the third power of gestational age. The result is a description of placental weight as a modified quadratic function of the time of gestation. The descriptive equation gives a reasonable fit to literature dataonplacental weights. A possible approach to including a term that is cognizant of the weight per centile of the fetus or placenta was discussed. Growth of the monkey placenta cannot yet be adequately fitted to these relationships because of limited literature data; possible descriptive equations were given. An approach to describing the thickness of the placenta, as the ratio of the weight to the surface area, was proposed.