Hey girls! Based on ultrasound and my cycle, we're setting up an appoi…

Hey girls! Based on ultrasound and my cycle, we're setting up an appointment for December 10th. My cycle is 35 days long and ovulation is always late because of that. My last period was in late February, and we conceived when ovulation was around March 16 or 18. So, it will be 9 months full by December 20th. I'm calculating on December 20th! Ultrasound and cycle on December 10th. Or am I doing this wrong? ??