Why did families have more children per family in the past?

In the past, women had five or six children on average, though many babies died soon after birth or in their infancy. For example, in the eighteenth century in France, half of all children died before age ten, and only one-third reached adulthood. Parents had lots of children to ensure that at least some of them survived. Today, child mortality is much lower and in many countries the death of a child is a rare event.