Why is it necessary for the process of mitosis to undergo two rounds of division?

Is it meiosis??
Because mitosis only undergoes one round of division resulting in 2 daughter cells {which are identical}
-To produce 4 daughter cells (haploid) 
Cells undergoing mitosis just divide once because they are forming two new genetically identical cells where as in meiosis cells require two sets of division because they need to make the cell a haploid cell which only has half of the total number of chromosomes

They get pregnant at younger ages

The zone your looking for is the intertidal zone.