Accuracy of ultrasound in dating pregnancy

Patient: I have just found out that I am pregnant when taking a test n 23rd January. It said I am pregnant 2-3 weeks. This would work out right as I had sex with my partner during the time inwould have been ovulating. My last period was normal on the 24th december. I had sex with another man which my partner is aware of which was a mistake as we’ve been together a long time. I slept with this other person on the 3rd of December and I had my period as I said on the 24th. How accurate are scans to know how many weeks I am. Although it sounds like there is no way the other man could have got be pregnant I want to have this confirmed as I’m worried thank you

Doctor: I do understand your concern. Ultrasound dating is quite accurate and the difference may only be ┬▒ 5 days from the esti imated date of gestation. Earlier scans are also more accurate. From the results it certainly seems that your partner is likely the father of the baby. I wish you well, take care.