Analysis of maternal-fetal weight relationships in primates based on the regression equation F (fetal weight) = b · M (maternal weight)a indicates that prosimian and anthropoid primates are separated by an allometric shift which can be interpreted as a result of differences in placental structures. The regular occurrence of multiple births in both prosimians and anthropoids is limited to small-sized species only, and is considered to be the result of selective pressure against oversized dimensions of the fetus at term to guarantee successful delivery. Twinning in callithricids is demonstrated as having developed secondarily as a byproduct of evolutionary miniaturization, rather than as a retention of a primitive mammalian feature