This paper uses birth certificate data to provide novel estimates of the age-specific risk of a low birth weight birth (LBW, an infant born weighting <2500 g) for U.S.-born non-Hispanic Black and White mothers, and finds that patterns vary markedly over space and time. Notably, risk of an LBW birth for Black mothers increased much more steeply with age in 1991–94 than in 2014–17. This decline in LBW risks among older Black mothers led to a decline in the Black-White LBW gap of more than half a percentage point. Both patterns and changes were regional; while age gradients on the Black-White LBW gap were lowest in the South in 1991–94, by 2014–17 they had increased in the South and declined in the rest of the country. These descriptive data allow a new examination of hypotheses regarding the causes of age-specific racial LBW gaps. Research has found that racial disparities in a number of health outcomes, including LBW, increase with age, leading some to speculate that this increase is due to the cumulative effects of exposure to disadvantage. The large degree of variability in Black-White LBW disparities suggests that age-specific causes may also play a role. A series of counterfactual trend analyses explore the roles of two specific mechanisms, smoking and hypertension, and compares these to a more fundamental indicator of socioeconomic status: education. •This paper examines changes in Black-White birth weight disparities over space and time using US birth certificate data.•Disparities in low birth weight declined from 1991-94 to 2014-17, with gaps narrowing by more than 20% at some ages.•Almost all of this decline occurred at older ages (25-39).•These changes were much less prevalent in the South, where disparities had been smallest in 1991-94.•Changes in both cumulative and age-specific factors likely contributed to these declines.