Due date question

I'm confused. According to my last period I'm due Jan 1st. When I went and had my ultrasound done they said I may ovulated earlier because I have a short cycle and I was measuring about 4 days ahead. Instead of being 10 weeks I measured 10 weeks 5 days. They're keeping my same due date though. How should I count how many weeks I am now? I want to go have a gender reveal ultrasound done at a different office and they said I can come in the 15th weeks. Just curious as to when I'll even be 15 weeks lol