In the past, children asked their mothers how children were born. Now, children ask their mothers how to avoid having children. The joke is that

This means that in the past, children were basically only children, and the children were very happy. He would ask his mother how he gave birth to him, and the child would be proud of it.

Now that the country encourages having two children, a family is no longer just one child. The child will worry that he will no longer be favored. Therefore, the selfishness in the heart will prevent the mother from having another child.