A ratio is an expression of the relationship between a numerator and a denominator regardless of the population from which the numerator and denominator are derived. A ratio, therefore, is simply the value obtained by dividing one quantity by another.
Since it is not possible to calculate accurately maternal mortality rate because it is not practicable to measure all pregnancies which is the denominator for rate, we have accepted maternal mortality ratio as its proxy. Even then some authors still use the terms (rate and ratio) interchangeably. Maternal mortality ratio is calculated by dividing the number of maternal deaths regardless of pregnancy outcome (ectopic pregnancy, fetal deaths, live births) by the number of live births in a defined population1. The question that has not been satisfactorily answered to date is: why were live births chosen as the denominator and not total births or total maternities? The argument that, when live births are used as the denominator the ratio becomes a rate (and not a ratio) is statistically faulty!
Apart from the statistical appropriateness of using total births or maternities as the denominator for calculating maternal mortality ratio, there are other merits. Maternities are the number of mothers delivered of registrable live births at any gestation or stillbirths of 24 weeks of gestation or later. These are the majority of women at risk of death from obstetric causes2. Irrespective of the level of sophistication of a community or a health institution, it is much easier and more reliable to obtain data on total births for a given period than live births. Equity is another reason why we should use total births as our denominator. Because of the less than satisfactory prenatal and intrapartum care that pregnant mothers receive in most developing countries, there are more cases of fetal deaths or stillbirths than what occur in countries with sophisticated health care. As a result, while live births almost approximate to total births in developed countries, the difference between the two in developing countries could be enormous. However, because we have been made to believe that maternal mortality ratio should be calculated using live births as the denominator, the relatively smaller number of live births (as opposed to total births) will inevitably lead to an outrageously high maternal mortality ratio in impoverished communities or health institutions3. Most earlier studies on maternal mortality used total births or maternities as the denominator. It therefore becomes difficult to compare the results of such studies that used total births with more recent works that use live births as the denominator.
My opinion is that we should go back to where we started. We should continue to use total births or maternities as our denominator for maternal mortality ratio instead of live births. Total births or maternities as the denominator for maternal mortality ratio is reliable, realistic and right!