How often does a birth of a baby occur in the us?

Slightly fewer than 4 million babies are born in the United States each year, and the details of how, when, and where they arrive are always shifting. Total fertility rate is an estimate of the average number of births a group of women have over their lifetime. The U.S. fertility rate has been declining since 2007.

the birth rate in the united states is 11.6

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.