CONTEXT: Despite the fact that most maternal deaths are preventable, maternal mortality remains high in many developing countries. Target A of Millennium Development Goal (MDG) 5 calls for a three-quarters reduction in the maternal mortality ratio (MMR) between 1990 and 2015. METHODS: We derived estimates of maternal mortality for 172 countries over the period 1990-2008. Trends in maternal mortality were estimated either directly from vital registration data or from a hierarchical or multilevel model, depending on the data available for a particular country. RESULTS: The annual number of maternal deaths worldwide declined by 34% between 1990 and 2008, from approximately 546,000 to 358,000 deaths. The estimated MMR for the world as a whole also declined by 34% over this period, falling from 400 to 260 maternal deaths per 100,000 live births. Between 1990 and 2008, the majority of the global burden of maternal deaths shifted from Asia to Sub-Saharan Africa. Differential trends in fertility, the HIV/AIDS epidemic and access to reproductive health are associated with the shift in the burden of maternal deaths from Asia to Sub-Saharan Africa. CONCLUSIONS: Although the estimated annual rate of decline in the global MMR in 1990-2008 (2.3%) fell short of the level needed to meet the MDG 5 target, it was much faster than had been thought previously. Targeted efforts to improve access to quality maternal health care, as well as efforts to decrease unintended pregnancies through family planning, are necessary to further reduce the global burden of maternal mortality.