Having a baby, having your full time off, coming back for a month then announcing you're 4 months pregnant and will be off again. If you knew you were pregnant (or even trying) should you really go back to work, knowing that your employer was going to have to pay two salaries for one job?
I really don't know if this is standard practice and completely ok or whether it's unfair and cheating the system. It seems unfair and a bit immoral to me.