I would suggest that the reward per baby be capped at some value, while the passive income have a cost that increases with every purchase. Even if it is just +1 cost per level, it would prevent the run-away growth that's possible. And capping the value per baby is just for logical consistency: no matter how healthy the baby is, it doesn't make any sense that someone would pay 10k each when they start out at so much less than that.
As it is, I spent all of my cash from the first birth on passive income, continued to level that until it was decently high, then spent the rest on baby value until the second birth, then immediately dumped all of the cash from that birth into guaranteed embryos, spent all of my passive income during the third pregnancy upgrading baby value, and then got so much money from the 3rd birth that I saw no reason to keep playing, as it would take a few minutes to spend all of the cash I got from that birth.
Quite fun, and definitely something there, but it needs a balance pass.