The fertility rate and the natality birth rate are two important metrics used in demography, but they measure slightly different aspects of population growth.
Fertility Rate: This is a measure of the number of live births per woman of childbearing age (usually defined as ages 15 to 49) in a specific time period, typically expressed per 1,000 women. It provides an understanding of how many children women are having on average in a population. A common way to gauge fertility is the Total Fertility Rate (TFR), which estimates the average number of children a woman would have throughout her lifetime based on current birth rates.
Natality Birth Rate: This refers to the number of live births in a population during a specific time frame, usually expressed per 1,000 individuals in the total population. Unlike the fertility rate, which is focused on women of childbearing age, the natality birth rate takes into account the entire population, thus giving a broader picture of birth rates regardless of the age or sex of the individuals.
Key Differences: The crucial difference between them lies in their focus. The fertility rate is specifically concerned with women in the childbearing age group and their reproductive behavior, while the natality birth rate captures the overall births in relation to the entire population. Therefore, a population with a high fertility rate might not necessarily have a high natality birth rate if there are fewer women of reproductive age, and vice versa.