Hidden twin

Found out last night we had a hidden twin that wasn't picked up on in my first 3 scans. I am in shock right now! I don't know why since twins run in ever generation on my fathers side. My mom has it on hers and my father in law is a twin. My sisters are twins. I even have twin niece and nephews. I am having identical twins! I can't believe it 😮 the doctor said at all 3 previous scans there was definitely only one! Guess doctors can be wrong sometimes