**Core Concept**
The maternal mortality rate (MMR) is a crucial indicator of the health care system's effectiveness in preventing maternal deaths. It is defined as the number of deaths due to pregnancy-related causes per 100,000 live births.

**Why the Correct Answer is Right**
The MMR is expressed as a ratio of the number of maternal deaths to the number of live births, usually presented as a rate per 100,000 live births. This allows for comparison of maternal mortality rates across different populations and settings. The World Health Organization (WHO) uses this definition to track and report MMR globally.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because MMR is not measured as a proportion or percentage of maternal deaths.
**Option B:** This option is incorrect because MMR is not expressed as a rate per 1,000 live births.
**Option C:** This option is incorrect because while MMR can be influenced by various factors, such as access to healthcare and socioeconomic status, it is not a measure of these factors themselves.

**Clinical Pearl / High-Yield Fact**
The MMR is a critical indicator of a country's or region's ability to provide quality maternal health care. It is essential to track and monitor MMR to identify areas for improvement and to ensure that policies and programs are effective in reducing maternal mortality.

**Correct Answer:** D. rate per 100,000 live births