These are the possible correct answers; Firstly, They have come to trust their parenting skills, secondly, they have fewer resources to "share" amongst multiple children and lastly, they see that the first-born child is developing in a healthy way.
Parents tend to relax in hopes of expecting later born children, since they have already saw their parenting skills in their first-born, wherein their parenting was tested, as they see their effect on parenting, it boosts their confidence that they can raise children properly.