Baby is 2 weeks behind

Just wandering if this is normal or if anyone else has had this experience. I went for a 4D scan this morning and the woman said that our baby is weighing/ measuring at 28-29 weeks. I am now 31 weeks pregnant is this normal should I be worried? I just want everything to be a nice normal pregnancy but so far it's been hell. Any advice would definitely be appreciated ! Thank you all !