My first day of my lmp was march 26 for 7 days. I slept with guy A April 1-7th. Then I slept with guy B on April 13th. My due date is jan 6th. I'm really scared I don't know who the father is? Going by my lmp my due date should be dec 31. Going by my ultrasounds and the doctor my due date is jan 6. But that puts me ovulating on day 19..
Don't focus on a due date.The problem with due dates is that they are ALL an approximation. For years doctors added 40 weeks to your LMP START date to come up with a due date. That would be Dec 31.
Now we use ultrasounds. Again, the due date is an approximation. No one can tell you exactly when you conceived AFTER the fact. It is not immediate and the timing varies. Fetuses develop at different rates/times too.
With a 28 day cycle you would be expected to ovulate sometime between days 11 and 15. You had sex with Guy A on days 6-12. Sperm can survive up to about 5 days max. You likely had sex at/near your peak fertility. Guy B is likely too late to be the father. You had sex with him on day 18. It's still possible - your egg can survive up to about 48 hours max - but not very likely.
Odds favor Guy A being the father. Only a DNA test can confirm.