**Core Concept**
Crude birth rate (CBR) is a fundamental demographic indicator used to assess fertility levels within a population. It represents the number of live births occurring in a given year per 1,000 people in the population, typically expressed as a rate.

**Why the Correct Answer is Right**
The crude birth rate is calculated by dividing the total number of live births in a year by the mid-year population, usually expressed as a rate per 1,000. This method ensures that the denominator accurately reflects the population at risk of giving birth during the year, providing a more accurate and representative measure of fertility. The mid-year population is used to account for fluctuations in population size throughout the year.

**Why Each Wrong Option is Incorrect**
**Option B:** The total population is not used in the calculation of crude birth rate, as it would overestimate the number of births and lead to an inaccurate representation of fertility.
**Option C:** While live births are indeed included in the numerator, the crude birth rate calculation also requires the mid-year population as the denominator for accurate representation.
**Option D:** Pre-term births are not specifically included or excluded in the calculation of crude birth rate, as the focus is on live births occurring within the given year.

**Clinical Pearl / High-Yield Fact**
When calculating demographic indicators, it's essential to use the most representative population figures to avoid biases and inaccuracies. In this case, using the mid-year population provides a more accurate picture of fertility levels within a population.

**✓ Correct Answer: A. Mid year population**