Not only having fewer kids, but if every generation doesn't have kids until they are in their forties they are skipping a generation and that means they would have to have five kids to maintain the numbers.
A hi vavasati hinkwavo va lavaka vana, hikwalaho ke hambanani na ku vula vana va vanhu timhika nova hlayelela leswaku hi nkarhi wo nge ava fane se vari na vana.