Eh, tak perlu risau lah! The 12-week scan is actually the most accurate one for dating, that's when they can pinpoint EDD paling betul based on baby's size. So kalau your baby dah measuring 13 weeks at the 12-week scan, it just means either your conception date was slightly earlier than the first scan suggested, or baby's growing a bit ahead of schedule (which is totally normal lah).
Kena tanya doktor on your current situation. During my first pregnancy pun I ada 3 EDD and every time RE-EDD gynae kata normal.
Kalau beza dlm seminggu je masih okay sis