**Core Concept**
Maternal mortality rate is a measure of the number of deaths due to pregnancy-related complications per 100,000 live births. It is an important indicator of the quality of obstetric care and the overall health status of a population.

**Why the Correct Answer is Right**
The maternal mortality rate is expressed as the number of deaths due to pregnancy-related causes (such as hemorrhage, sepsis, or hypertensive disorders) per 100,000 live births. This ratio provides a standardized way to compare the risk of maternal mortality across different populations and settings. The World Health Organization (WHO) recommends using the maternal mortality ratio (MMR) as a key indicator of maternal health.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the maternal mortality rate is not expressed as a proportion of the total number of births or the number of women of childbearing age.

**Option B:** This option is incorrect because the maternal mortality rate is not expressed as a ratio of deaths to the total number of pregnancies.

**Option C:** This option is incorrect because the maternal mortality rate is not expressed as a rate of deaths per 100,000 population.

**Option D:** This option is incorrect because the maternal mortality rate is not expressed as a proportion of the number of women who give birth in a given year.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the maternal mortality ratio (MMR) is a more accurate measure of maternal mortality than the maternal mortality rate (MMR/100,000 live births), as it takes into account the number of live births and not just the number of deaths.

**Correct Answer: C. Number of deaths due to pregnancy-related causes per 100,000 live births.**