My last period was on the 16th of Dec and ended the 20th. Iv been having regular unprotected sex for months with him and nothings happened, I have previously fell pregnant by him although it took 8 months of trying. We was having sex from the 16th Dec continuously til the 2nd of Jan, then stupidly I had a one night stand on the 9th Jan... Who is more likely to be the father? I took a test on 25th Jan and it was positive, currently 27 weeks pregnant. Please help.
That will mean that your doctor thinks that you ovulated/conceived around Dec 30. That will mean that intercourse around Dec 26 to Dec 31 could have gotten you pregnant. At 12 weeks the scan can be off by as much as 5 days. That will move the latest date for baby making sex to Jan 6 and the earliest to Dec 21.
With your LMP date it however looks like the Dec 30 date is more or less correct. So Mr ONS on Jan 9 is very very unlikely to be the father of your baby.