Are dating scans accurate?

Patient: My scan gave me a due date of 22nd january 2012. If I count back 38 weeks it gives me a conception date of 1st may. Me and boyfriend split up and I had a one night stand on 5th april. I then had bleeding on 15th april like a period, heavy lasted 3,4 days with cramps. I have heard of implantation bleeding but this was just like any normal period to me.. Me and boyfriend then got back together and had intercourse on 1st may. There’s like a 3,4 week difference between the 2 men is this enough to tell who’s the father? The dates match up to my boyfriend on 1st may. The baby measured 5.8cm at 12 weeks 2 days. But we were trying for a baby for 2 years and I never got pregnant. As its funny how I sleep with someone else and get pregnant even though I have a period and dates don’t match to him. So could this be the one night stands or ex boyfriends and I finally conceived by my boyfriend?! My boyfriend knows and is really happy as we have been wishing for a child for 2 years! He doesn’t know about the one night stand should I say anything or is it more likely his? Can a dating scan put me out by nearly 4 weeks? Can a 16 week old baby measure 5.8cm? If it is one night stands baby I am now nearly 39 weeks pregnant not nearly 35. So if I give birth on or after due date of 22nd jan I will be 34 weeks pregnant? Is that possible?

Doctor: Ultrasound dating scans are fairly accurate if done early in the pregnancy. A scan done after 20 weeks of gestating may show variability and sometimes may indicate if the baby is not growing at a rate it should. The accuracy is also determined by the machine and the skill of the operator. I would advise you to speak with your doctor to determine the best possible date of conception. I hope the information helps, take care.