Tough position

So the father of my baby and I work together we have been in a relationship for a year now. Theres nothing but girls at work there besides him and my manager. My manager knows we are together and he doesn't care as long as we are staying focused when it comes to work. The girls think there's something going on but we have not confirmed it cuz they're so judgemental. I know I need to tell my job as soon as possible before I start to show please help me on how to telling the team. My boyfriend thinks I should say that I don't know who the father they said they don't question me. Any ideas on how to tell them?