Maternal mortality rates are computed on the basis of the number of live births. The maternal mortality rate indicates the likelihood of a pregnant woman dying of maternal causes. They are calculated by dividing the number of maternal deaths in a calendar year by the number of live births registered for the same period and are presented as rates per 100,000 live births. The number of live births used in the denominator is an approximation of the population of pregnant women who are at risk of a maternal death. “Maternal deaths” are defined by the World Health Organization as "the death of a woman while pregnant or within 42 days of termination of pregnancy, irrespective of the duration and the site of the pregnancy, from any cause related to or aggravated by the pregnancy or its management, but not from accidental or incidental causes." Included in these deaths are ICD–10 codes A34, O00-O95, and O98-O99. A description of the maternal mortality measurement has been published by NCHS.
Estimates were calculated with data supplied by the states that had adopted the 2003 version of the U.S. State Certificate of Live Birth. Trends over time cannot be evaluated until such time as all states provide data in the same format.
Twenty-two states had implemented the 2003 birth certificate as of January 1, 2007. The 22 revised states represent 53% of all U.S. births.