Paternity Testing & Pregnancy Concerns After Multiple Partners

Question
My boyfriend of 4 years and I broke up September of last year and I hooked up with a new guy who I dated for 3 months (Sept-Dec.). In December my boyfriend and I got back together. I had sex with the other guy for the last time November 28th. However, the most that couldve came from him was pre-cum. My boyfriend and I had sex multiple times in December and beginning Jan. My cycles are EXTREMELY irregular and I was inconsistently taking birth control pills. Now, I am scared because I had a UTI at the beginning of Dec. and I know that is a sign of pregnancy. However, I also had a longer than normal period that was HEAVY (as normal) and it began on time Dec.1. My period was also longer and HEAVY in Nov. Due to the inconsistancy of the pills. After I had a light spotting for 1 day at the beginning of Jan. and my period never came I decided to take a amilyDollar home preg test Jan. 13 that came back positive with a light line. On Jan.18 I had a transvaginal ultrasound becuase the transabdominal wouldnt pick up dating me at 4 weeks 5 days measuring 7mm, Feb. 16 I had another ultrasound that dated 8weeks 4 days measuring 20 mm. Both u/s giving a due date of Sept.23 Late May we had our 21 week ultrasound but it changed the due date from the 23 to the 18th of Sept. My baby was born Sept. 22 and my boyfriend believes and wants to be the father but wants a DNA test to verify paternity. I am worried terribly and need to know if my bf is the fateher.How accurate are my u/s?

Answer
Dear Tiffany,
Based on the dates you gave me, it is extremely unlikely that you conceived on November 28th.
Even if the correct due date was September 18th, your date of conception would have been between December 19 and December 26. Also, the description of your periods indicate that you got pregnant in late December.
The early ultrasounds are quite accurate. The later ones can be off a bit sometimes. But since you had the earlier ones, I would say you can be pretty sure.
Of course, I can't give you an answer 100%, since the dates are close, but it is highly probable that your current boyfriend is the father.
Best of luck,
Sheva
PS A UTI is sometimes a result of pregnancy, but can also be a result of having a new partner - so this may be the reason you had a UTI in early December.