**Core Concept**
The maternal mortality rate (MMR) is a measure of the number of deaths among women due to pregnancy-related complications per 100,000 live births. It is an essential indicator of the quality of maternal health care in a given population.

**Why the Correct Answer is Right**
The denominator for the maternal mortality rate is the number of live births, as it reflects the population at risk of maternal mortality. Live births are births where the fetus has a heart rate and is breathing at the time of birth, regardless of the gestational age. The World Health Organization (WHO) recommends using live births as the denominator for calculating the maternal mortality rate. This is because live births are a more accurate representation of the population at risk of maternal mortality compared to other denominators such as the number of women of reproductive age.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the number of pregnancies is not the correct denominator for calculating the maternal mortality rate. The number of pregnancies includes both live births and fetal losses, making it an inaccurate representation of the population at risk.
**Option B:** This option is incorrect because the number of women of reproductive age is not the correct denominator for calculating the maternal mortality rate. This number includes women who are not pregnant and are not at risk of maternal mortality, making it an inaccurate representation of the population at risk.
**Option C:** This option is incorrect because the number of stillbirths is not the correct denominator for calculating the maternal mortality rate. Stillbirths are births where the fetus has died in the womb and is not breathing at the time of birth, making it an inaccurate representation of the population at risk.

**Clinical Pearl / High-Yield Fact**
The maternal mortality rate is a critical indicator of the quality of maternal health care in a given population. A high maternal mortality rate indicates poor access to prenatal care, inadequate emergency obstetric care, and a lack of skilled birth attendants.

**Correct Answer: D. Live births. Live births are used as the denominator for calculating the maternal mortality rate.**