To cancel maternity leave with AM - type "backtowork" in the console. I can't speak to your length-of-leave issues, but if you are on maternity leave and get pregnant again it will give you more time added to the current leave. The leave is the number of work days, so if the job is only three days a week and you get four days of leave the sim will be off for over a week. I had an astronaut get pregnant and get about a month off of work.
Just something that doesn't make sense to me though, why would that make 4 days into 12 days? It's not like you can work out 4 days into 3 weeks, cause that would mean it would be like 2 days of work for the week, then 1 day of work for the week, then 1 day of work for the week, which, as far as I know, doesn't happen.