Measurements?

When I went for my 12 weeks scan they told me I was 12 weeks and 5 days so when I went for my 20 week scan I was 19 weeks and 6 days. The women told me that his arms and legs (I think that's what she said, I was to busy watching my boy on screen) are measuring 19 weeks but the rest of his body was measuring 20 weeks and 4 days, so she said to keep my due date as 10th of February, is there a chance I'm further on then I think?