**Core Concept**
The maternal mortality ratio (MMR) is a key indicator of maternal health and is defined as the number of maternal deaths per 100,000 live births. It is an essential tool for monitoring the quality of maternal healthcare and identifying areas for improvement.

**Why the Correct Answer is Right**
The correct denominator for calculating the MMR is the number of live births, as it is a direct measure of the population at risk. Live births are defined as births with a minimum gestational age of 28 weeks or a birth weight of 1,000 grams or more. The use of live births as the denominator helps to standardize the MMR across different populations and time periods, allowing for more accurate comparisons and trends to be identified.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because fetal deaths (stillbirths) are not included in the denominator for MMR calculations. Fetal deaths are a separate category and are used to calculate the fetal mortality rate.

**Option B:** This option is incorrect because the total number of births (live births + fetal deaths) would overestimate the denominator, leading to an artificially low MMR.

**Option C:** This option is incorrect because the number of women of childbearing age (15-49 years) is not a direct measure of the population at risk for maternal mortality.

**Clinical Pearl / High-Yield Fact**
The MMR is a lagging indicator of maternal health, meaning that it reflects the quality of care provided during the preceding year. As a result, improvements in maternal health and mortality rates may not be reflected in the MMR until several years after the implementation of new policies or interventions.

**Correct Answer: D.**