That nurse is correct. The early ultrasounds are the most accurate when it comes to dating.
However, even though your period might come like clockwork, ovulation does not always work the same way. Note that I said
always.
Ultrasounds can be off as much as two weeks, even that early.
I'll try to explain it the way the OB I work for explains it to his patients...
"Unless there is a camera in your fallopian tubes we don't have any idea exactly when you conceived as we can't see exactly when the sperm met the egg. All of obstetrics is a guessing game based on educated scientific formulas. All dating whether it be by LMP or ultrasound can be off as much as 2 weeks either way."
I myself have delivered babies at 36 weeks gestation who have turned out to be full term babies as justified by something called a
Dubowitz Score. I have also delivered full term babies (37 weeks and above) that would have a Dubowitz Score of a 34 week gestational baby.
Look, I'm not trying to be a Debbie Downer on you. It's just that you have to face reality in that there is a possibility, no matter how remote, that either man could be the father.
Another little known fact, did you know that twins could come from two different fathers?