at the time I thought I was 6W 3 days , but when I had my ultrasound baby measured at 7 weeks 3 days which was also my lmp. it confuses me cuz I didn't even have sex that day , more like a week after that. so could the ultrasound be a little off?
It confuses me also but I am 18wks and today they measured my little guy at 19wks instead. I also didn't have sex around that time, but the tech last says it varies by nine days and that my doctor will still keep the same due date and everything, and that I don't need to change anything, baby is just growing a little faster than normal. It all kind of depends but your due date and everything should stay the same!
The baby isn't made the day you have sex, it can take several days. So just like everyone else said they go by first day of last period.