Correct use of the term “Taxa de Natalidade” [closed]

Birth rate is correct — see, for example, the Cambridge Dictionary definition:

Birth rate the number of births that happen during a period of time in a particular place

The number of births per unit time (e.g. per year) is a rate (see, for example, the definition in Merriam Webster).

Rate 1 a : a quantity, amount, or degree of something measured per unit of something else

(I would say Merriam Webster’s “something else“ should be time.)

As a biological scientist, I would say that your original “fertility rate” is scientifically incorrect, as fertility is a state:

Fertility the quality or state of being fertile

However fertility it is misused by the media to mean birth rate, as reflected in the second entry for Merriam Webster.

One diminishes the power of the English language to discriminate between objects and ideas by accepting this sort of misuse†. However, if you really must, the logically correct form of this would be fertility without the rate.

† A decrease in the birthrate can be caused by a number of factors, including increased use of contraception, lower frequency of sexual intercourse, and decreased fertility. Fertility would be measured by the frequency of pregnancy resulting from sexual intercourse when other factors have been controlled.