I like my couples to have as many children as possible. Just because a persons preference is "Definitely wants children, No Way", etc doesn't mean that baby making will be any easier. And just because a doctor consult doesn't tell you the first time that a person might need baby boost if they want to have children doesn't mean they won't ever need it. One of my couples had several children and suddenly the "need" for baby boost came up in one of the consultations. Weird.

When my couples try to make a baby and are unsuccessful, In one of my paused games I have over 32 generations.