This longitudinal study investigated neurobehavioral development in the human fetus from 24 to 36 weeks gestation. Subject (N=103) were stratified by socioeconomic class. Fetal data were collected for 50 min at three intervals, and included measures of heart rate, movement, and biobehavioral patterns. Repeated measures analysis of variance by fetal sex and maternal socioeconomic status was used to detect maturation effects and group differences. With advancing gestation, fetuses exhibited reduced heart rate, increased heart rate variability and coupling between movement and heart rate, increased movement vigor, and more biobehavioral concordance. Male fetuses displayed higher heart rate variability throughout gestation and somewhat earlier emergence of biobehavioral organization than females. Fetuses of women of lower socioeconomic status had reduced heart rate variability, moved less often and with less vigor, showed less coupling between movement and heart rate, and had fewer episodes of synchronous quiescence/activity. Results are discussed in terms of development of the central nervous system.