Tak pasti berapa minggu

Hi, 20/1/2022 upt positive and based on my first day of my last period (13/12/2021) my baby shud be within 5-6 weeks tapi masa scan doc kata belum nampak apa apa lagi. Dah check for cyst and fibroid but also clear, doc consider might be baru lagi in 2 weeks macamtu. Possible ke? So macam mana sebenarnya kiraan usia pregnancy?