Hello! I marked the last day of critical days in this app and it set m…

Hello! I marked the last day of critical days in this app and it set me up for 6 weeks of pregnancy. Today at ultrasound they said 4 weeks!!! ? On Wednesday there was nothing visible on ultrasound, but today there was clearly 3.7 mm, they gave me a photo. And the term was set to 4 weeks. So 4 or 6 and why does it indicate that 6 weeks here? ? Can you please tell me what your thoughts are on this?