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Hi It's Dr. C. Board certified Ob-gyne. I can help you today.
Congratulations on your pregnancy. I'm sorry that there is some confusion about who the father is though. That's a tough situation. In this case, because the sex dates are only 4 days apart, it's not possible to tell without paternity testing.
The reason for this is that ultrasounds are the most accurate way to tell conception date, but they have an error rate of 3 days even if done in the first 3 months of pregnancy. On top of that, sperm can live inside a woman for 3-5 days. So even if the ultrasound shows a conception date of March 16, it's still possible that there was some sperm left inside the vagina from the 12th.
Paternity testing would require just a blood sample from you and a swab or blood from just one of the men. You'll want to use a lab that is legally certified by the state that you live in. This information is usually available on their website by typing "Paternity testing" into their website search engine.
I hope it all turns out well for you. Please reply if I can clarify anything or add details. I know this is a very tough situation to be in. Again, I wish you good luck.