Dating scan

Ada mummy2 yg ada pengalaman tak, period tak regular. First scan tak nampak baby, cuma nampak kantung. Doctor kata baru 5 weeks. Lepas 2 minggu repeat scan, nampak baby dgn heartbeat tapi tetibe dah 8 weeks 2 days. Kalau ikut lmp dah 10 weeks. Macam mana tu