There's no option to raise baby normally because I dont think anyone would spend tens thousands of days of waiting for them to grow up.
Viewing post in Old Bug Report Thread
I always thought the maturity was the wait, and the instant was an option to immediately (or maybe after a day) grow up in exchange for a higher price. I know it's difficult to put down a good balance of what days represent, which is why I'd made the suggestion of adding it as a constant like the pregnancy time.
Also, I've currently got the pregnancy set to 22 days, and my headgirl's updates on the pregnancies are behind. Can you tweak that in the future so that it reflects the stages better? She literally gave birth and then my headgirl told me that said slave was likely to give birth soon.