Introduction
The importance of quantifying the loss of life caused by maternal mortality in a population is widely recognized. In 2000, the UN Millennium Declaration identified the improvement of maternal health as one of eight fundamental goals for furthering human development. As part of Millennium Development Goal 5, the UN established the target of reducing the maternal mortality ratio by three-quarters between 1990 and 2015 for all national and regional populations.1
The maternal mortality ratio (MMRatio) is obtained by dividing the number of maternal deaths in a population during some time interval by the number of live births occurring in the same period. Thus, the MMRatio depicts the risk of maternal death relative to the frequency of childbearing. A related measure, the maternal mortality rate (MMRate), is found by dividing the average annual number of maternal deaths in a population by the average number of women of reproductive age (typically those aged 15 to 49 years) who are alive during the observation period. Thus, the MMRate reflects not only the risk of maternal death per pregnancy or per birth, but also the level of fertility in a population.
In addition to the MMRatio and the MMRate, the lifetime risk, or probability, of maternal death in a population is another possible measure. Whereas the MMRatio and the MMRate are measures of the frequency of maternal death in relation to the number of live births or to the female population of reproductive age, the lifetime risk of maternal mortality describes the cumulative loss of human life due to maternal death over the female life course. Because it is expressed in terms of the female life course, the lifetime risk is often preferred to the MMRatio or MMRate as a summary measure of the impact of maternal mortality.
However, despite its interpretive appeal, the lifetime risk of maternal mortality can be defined and calculated in more than one way. A clear and concise discussion of both its...