I think my doctor is wrong

I'm pretty confident that I know what day I got pregnant because I logged my periods before my pregnancy. He tells me I'm 26 weeks but I'm pretty sure I'm 24 weeks. This causes problems because when I had my anatomy scan they were saying that my baby measured small but I think it's because they were looking for a bigger baby (she's not as old as they think she is so she'd be smaller). Has anyone else experienced this??