By design if the child is not the player's, then start with 18 age (my guess because the 'childcare' action chain).That is the main difference, the game does checks if you are the baby-daddy. But there is special scenarios, when player's girlfriend might end up pregnant by another npc, so scene has to account for that as well.
Also I reported long time ago in the bugfix channel (but why if this still unfixed?) a bug about the other condition there. The mother can be giving more birth than one child but this condition's syntax is wrong: 'Actor:fertility'. The 'fertility' parameter used exclusively the player, so it must be replaced to 'Actor:fertility_npc' to be correct and conditions has chance to be meets.