baby help

hey, I'm 16 and currently 5 months pregnant. I have been for ultrasounds, and stuff so it's definite 
But I haven't really gotten that big so no one has really asked!! But I haven't told my mum 😖 and the baby is due in 4 months 
I just don't know how to tell my mum!? My boyfriend has told his mum and dad, but I have no idea what to say!? Help