The part about ‘get used to it’ indicates to me that SAD was clueless. Real mothers are motherly and don’t hand off their babies at a whim, especially to make someone else do their dirty work. Doing that means SAD either had no confidence or no clue on how to handle the baby’s diapers. There’s no need to invent excuses.
One word - teenager.
There have been teenaged mothers throughout the ages. It’s less common now but that’s merely a cultural anomaly.
It looks to me - reading through the lines - that there was probably an attachment problem, as well as whatever else was weird in the timeline department.