was told by my doctor that if I wanted children I would need some "medical assistance", which was fair enough as I wasn't planning anything anyway. I became pregnant unexpectedly and now I'm unsure what to do.

On the week my period was due (which obviously didn't arrive) I slept with somebody other than my partner (wrong I know...we were actually going through the process of separating). When I went to the doctor a week later, she put me to be around 6 weeks already pregnant. At the 12 week scan it confirmed this.

Is there any possible way that I got pregnant later on and the ultrasound was wrong? I hope this makes sense. I have a normal cycle so I know in my mind that I would have been 12 weeks at my 12 week scan if my regular partner was the father and 9 weeks if not. The scan put me a spot on 12 weeks.

My 12 week dating scan indicated the baby measured exactly 12 weeks which would be correct for my dates and LMP. I have regular periods so know when my LMP was.
I was getting worked up because I wanted to know if the dating scan could be wrong by 3 weeks.

I have found a website on fetal measurements which say at 12 weeks the CRL should be 57mm which it was. If I took it from the date I would have conceived with the other man it would show up as only 9 weeks (or 7 not included the 2 weeks they add on) which would mean a measurement of 23mm (9 weeks) or 11mm (7 weeks)