I saw earlier in a different feed an app called Baby Story being used. The app bubble is white with a white B in a red circle. I downloaded it and played around with it and liked it.

When my ex and I were pregnant we used the oldest and best one out there. Our mouths!! Haha, or people saw my bump. Really, at the end of the day, your dear friends and family care and they all already know. No on is fooled for as long as people think they hide it. If you want to put something out there, any of the numerous apps are good, just make it simple and people will "like" it!