Human growth is a continuous process. Studies defining factors influencing growth focus on discrete time points (e.g., birth), overlooking the conditional nature of the process. One thousand six hundred fifty Caucasian mothers who gave birth at term after an uncomplicated singleton pregnancy were studied using conditional analysis. Infant height, weight, and head circumference were obtained at birth and 6 mo of age. Data analysis, conditional upon birth size, was conducted as a stepped consideration of factors influencing phases of fetal and infant growth beginning with determinants of placental size. Placental weight was related to birth size. Seven percent of the variance in placental weight was explained by a combination of gestation at delivery, maternal size at first prenatal visit, paternal height (all positive), and increasing parity (negative). When centered on birth weight, 41% of the variance in placental weight was explained by birth weight, length of gestation, smoking during pregnancy (all positive), and a female baby (negative). Maternal and paternal stature equally influenced newborn and infant size. Conditional analysis reveals a series of modifiable (parity, length of gestation, and smoking) and nonmodifiable factors at different stages of the growth process.