We examined trends in low birth weight (LBW, <2,500 g) rates among US singleton non-Hispanic black infants between 1991 and 2004. We conducted Joinpoint regression analyses, using birth certificate data, to describe trends in LBW, moderately LBW (MLBW, 1,500-2,499 g), and very LBW (VLBW, <1,500 g) rates. We then conducted cross-sectional and binomial regression analyses to relate these trends to changes in maternal or obstetric factors. Non-Hispanic black LBW rates declined -7.35% between 1991 and 2001 and then increased +4.23% through 2004. The LBW trends were not uniform across birth weight subcategories. Among MLBW births, the 1991-2001 decease was -10.20%; the 2001-2004 increase was +5.61%. VLBW did not follow this pattern, increasing +3.84% between 1991 and 1999 and then remaining relatively stable through 2004. In adjusted models, the 1991-2001 MLBW rate decrease was associated with changes in first-trimester prenatal care, cigarette smoking, education levels, maternal foreign-born status, and pregnancy weight gain. The 2001-2004 MLBW rate increase was independent of changes in observed maternal demographic characteristics, prenatal care, and obstetric variables. Between 1991 and 2001, progress occurred in reducing MLBW rates among non-Hispanic black infants. This progress was not maintained between 2001 and 2004 nor did it occur for VLBW infants between 1991 and 2004. Observed population changes in maternal socio-demographic and health-related factors were associated with the 1991-2001 decrease, suggesting multiple risk factors need to be simultaneously addressed to reduce non-Hispanic black LBW rates.