BACKGROUND: Low birth weight (LBW) can result from prematurity or intrauterine growth restriction (IUGR) and result in small-for-gestational-age (SGA) infants. Prematurity and IUGR may have different etiologies and consequences. OBJECTIVE: Our objective was to analyze seasonal patterns of prematurity and SGA in a rural African community and to compare them against variations in nutritional and ecologic variables that may provide insight into likely causative factors. DESIGN: Fourier series were used to compare the seasonality of prematurity (<37 wk) and SGA (<10th percentile of the reference standard) among 1916 live infants born over 26 y in 3 Gambian villages. The resultant patterns were compared against monthly variations in birth frequency, maternal energy status, maternal work, and malaria infections. RESULTS: The incidence of LBW was 13.3%, of prematurity was 12.3%, and of SGA was 25.1%. Prematurity and SGA showed divergent patterns of seasonality. Incidence of SGA was highest at the end of the annual hungry season, from August to December (peaking in November at 30.6%), with a nadir of 12.9% in June. Rates of SGA varied inversely with maternal weight changes. This pattern was not seen for rates of prematurity, which showed 2 peaks-in July (17.2%) and October (13.9%). The lowest proportion of preterm births occurred in February (5.1%). The peaks in prematurity closely paralleled increases in agricultural labor (July) and malaria infections (October). CONCLUSION: We conclude that a reduction in LBW in such communities may require multiple interventions because of the variety of precipitating factors.