Step 1: Consider the standard demographic ratio used to define fertility: $\text{Fertility Rate} = \dfrac{\text{Number of live births}}{\text{Female population of reproductive age (15-49 years)}}$.
Step 2: This shows fertility is a rate relative to the reproductive age group of females, since women outside 15-49 years cannot contribute births. Using total female population as the denominator would dilute and mis-measure fertility.
Step 3: Total live births form the numerator, not the base the rate depends on, and the number of pregnant women is an unobserved intermediate quantity not used in the standard definition.
Step 4: Therefore, the base population that fertility rates mainly depend on is the female population of child bearing age.
\[\boxed{\text{Female population of child bearing age}}\]