Is it normal that my mum kept her first pregnancy a secret from me?

Well me and my nana were talking about my mom in her living room, we spoke about this argument that my mum and dad had when they just split up (about 2 years ago when the argument happened) I stayed downstairs with them and my dad said, to my mum, "well she wasn't the first thing to come out of your womb!" And at the time my mum shrugged it off and it left me wondering what happened. For all my life i have been led to believe that I was, and to my knowledge, the only one they'd ever had so I just shrugged it off as an empty threat. So anyway a few minutes ago I asked my nana about what my dad could have meant and she said never to tell my mum that she told me but I wasn't their first attempt at a child, my mum and dad and managed to create a child but it was growing outside my mums tubes so she had to abort it in order to live, then they tried again and make me and I was born and everything was fine. Is it normal that she never told me? Is it bad that I always felt incomplete in some way? (I always felt like I wasn't the only one) why didn't they tell me? Is it bad that I wish they did have that baby?