first time pregnancy

J
I've had 3 positives on pregnancy tests and due to see dr on Monday. I didn't ovulate until really late in my cycle -on day 28. Then pee tested ten days later with a positive result and the lines were strong. Online calculators say I'm due 11th Dec so think I'm nearly 3 weeks - 5 if you count pre stage. Is it normal to show up as pregnant after only 10 days? My husband has twin cousins on his dad's side and his mum is a twin too. I've read somewhere a strong hormone early on could mean twins? Any ideas?