Still think I'm having twins!

I am 15w today and when I first found out I was pregnant, I instantly thought I was having twins. Twins run in my husbands family, so there is a huge possibility that it could happen. Scans showed just 1 sac and 1 baby (to my delight - twins scared me!) but I am already really big (it's my first pregnancy and everyone keeps asking if I'm 6 months pregnant!), and felt my baby kick at 14w 5days. Is it possible that there could be another baby hiding in there somewhere that we didn't pick up? Has anyone had this happen or know of someone that this has happened to before?