Clinically meaningful behavior problems are thought to be present beginning in the early toddler years, yet few studies have investigated correlates of behavior problems assessed before age 2 years. The current study investigated the direct and interactive contributions of early infant and caregiver characteristics thought to play an important role in the ontogeny of behavior problems. Specifically, the study examined: (a) the links between infant temperamental reactivity and toddler behavioral symptoms, (b) whether maternal sensitivity moderated associations between temperamental reactivity and behavioral symptoms, (c) whether variability in temperamental reactivity was explained by exposure to maternal stressful life events (SLEs) in utero, and (d) whether child sex moderated these pathways. Data were collected from 322 low-income, Mexican American families. Mother reports of SLEs were obtained between 23 and 40 weeks gestation; temperamental negativity and surgency at 6 weeks and 12 months; and internalizing and externalizing behaviors at 18 months. Maternal sensitivity during structured mother-infant interaction tasks at a 12-month visit was assessed by objective raters. Results indicated that significant paths linked maternal prenatal SLEs with 6-week negativity, 6-week negativity with 12-month negativity, and 12-month negativity with 18-month behavioral symptoms. Sex-specific effects were also observed. Maternal SLEs were directly associated with internalizing behaviors for girls only. Surgency and maternal sensitivity moderated the associations of negativity with subsequent externalizing behaviors for girls only. Results suggest that ecological stressors associated with sociodemographic risk factors such as low-income and ethnic minority status begin to exert cascades of influence on children’s developmental outcomes even before birth.