I had sex with my best friend who came down from the navy on 02-20-2015, me and my boyfriend got into an argument so at the time we weren't together. He went in without a condom for like 15 seconds not even close to a minute, then he put on a condom. So on March 13th or 15th me and my ex when back together, so from that day on we had unprotected sex multiple times. This June I found out I was pregnant, but my periods are irregular so I don't remember when I got my last period. We guessed and just said March 1st and that would put my due date as 12-6-2015. When I had my ultrasound the day after they said that the due date was still the same and this was at planned parenthood. So then I went to get an ultrasound somewhere else and they told me I was 2 days ahead from what PPH told me so my due date was 12-4-2015. So my concern is that my ultrasound may be wrong and it's my bestfriends. Please give me your input and help me out.

No way to know for sure. Ultrasounds aren't 100% accurate, it's a best guess based on the development of the fetus, and can be off as much as 2 weeks either way (so a month).

The only way to determine who the father is, is a DNA test after the baby is born.

I had sex with my best friend to came down from the navy on 02-20-2015, me and my boyfriend got into an argument so at the time we weren't together. He went in without a condom for like 15 seconds not even close to a minute, then he put on a condom. So on March 13th or 15th me and my ex when back together, so from that day on we had unprotected sex multiple times. This June I found out I was pregnant, but my periods are irregular so I don't remember when I got my last period. We guessed and just said March 1st and that would put my due date as 12-6-2015. When I had my ultrasound the day after they said that the due date was still the same and this was at planned parenthood. So then I went to get an ultrasound somewhere else and they told me I was 2 days ahead from what PPH told me so my due date was 12-4-2015. So my concern is that my ultrasound may be wrong and it's my bestfriends. Please give me your input and help me out.

Why would you think its OK for 15 seconds? It could happen the split second his penis touched your vagina... even without entry.

But long story short... you need a DNA test after the child is born to know who the father is.

When did you have the early ultrasounds? At what week gestation? Have you had any since? Are you about 22 weeks now?

There is no way to know conception. Two days difference is nothing, these are educated guesses. As noted before, dates of conception can be off as much as two weeks.

So while it is not as likely. Yes the "best friend" could be the father.

A DNA test is the only way to be sure, after baby is born.

When did you have the early ultrasounds? At what week gestation? Have you had any since? Are you about 22 weeks now?
My first ultrasound was at 16 weeks. I've had 4 so far and all have kept around the same date, I'll be 22 weeks tomorrow

Doesn't really matter... the fact is... there is too great a window to guess which it was... because that window is up to two weeks before that guess to two weeks after that guess.

Want to really tick off a guy... let him find out he's NOT the father after you were keeping the information that another man could be a secret from him.

If he knew up front... he would be far more understanding than finding out after the fact that you knew for 9+ months it might not be his.

Telling him anything less is being deceptive and beating around the bush....and would be the worst way to deal with this.

Early ultrasounds are the most accurate... actually within several days either side. Early second trimester, it bumps up a bit more. As pregnancy progresses they become less accurate, especially in the last trimester as individual growth differences will vary more. This is where you are more likely to see the discrepancy in weeks between sizing and gestation.

As others have said, to know for absolute certain, DNA testing would be necessary, but going with your several scans remaining consistent, your dates point to your boyfriend as most likely being the father. If they were off too far, due date would be changed. If scan results showed conception to be closer to Feb. 20, you would be 25 weeks now with a due date of mid-November.

Since you have concerns, best to let both men know is the possibility, see when baby is born, and have DNA testing done if there is any doubt.