By Aug 30th conception date, I meant if I was to have a 28 day cycle.
Did you get an early ultrasound, or was the 20th week the first ultrasound you had?
Also, could you mention what due date you were first given, and when, and what it was based on (an exam or an ultrasound)?
OK, let's look at what we know.
You were originally given the estimated due date of May 24. If that date were to be perfectly correct, the day you got pregnant would have been the previous August 31. Your doctor did an ultrasound at the 20th week, and said the baby was measuring two days bigger than previously assumed, which you are saying means that now the estimated due date is May 22. Since the presumed conception date for May 22 would have been August 29, you are somewhat concerned because you had sex on the 23 - 24 with your ex, and on the 30th with your friend. And sperm can live 4-6 days in your body (some doctors say 3-4 days and others say up to 7 days, though research suggests it might not be viable enough to penetrate the egg as late as day 6).
Here's some stuff to put into the hopper as you think about this.
First of all, if a doctor does an ultrasound and thinks a baby is two days bigger than originally noted on the record, quite often the doc won't change the due date. This is especially true if the ultrasound was not early in the pregnancy. Babies can develop faster or slower than average, but it doesn't change when they began. There would not be any point in changing a due date based on an ultrasound that is pretty far along in the pregnancy, when all the difference might mean is that the baby is a slightly fast grower. Your baby that is a tiny bit bigger than a baby conceived on August 31 could have still been conceived on August 31.
I get you're trying to definitively rule one guy out or in. The guy on the 23 - 24 is a less likely candidate, even if you were to think "but his sperm could have lasted until the 29th, when I might have ovulated." This is because your baby is a boy. By the 29th, the sperm from your ex would most likely all have been carrying the x chromsome, not the y chromosome. To know why, look up the Shettles method.
Shettles says the closer to ovulation you have sex, the higher the likelihood you will have a boy, because male sperm swim faster and die sooner than female sperm. The theory is that by about day 3 after sex, all the sperm carrying the y chromosome are dead. Since you know the baby is a boy, and it appears you had sex with the second guy right around when you ovulated (or a day later, which is still OK because eggs last 24-36 hours), the second guy seems to be the more likely candidate.
Of course, the margins for certainty here are hair-thin. If it is really important to know before the baby is born, do a prenatal DNA test with a reliable lab such as Ravgen or the DDC. In a perfect world, where men treat women like equals and take responsibility for their behavior, you and the two men would split the cost three ways (prenatal testing at a reliable lab is expensive) and both men would test, and you would know in a couple of weeks. (If you do this, be SURE both guys test.) A lot of the time, though, people just opt to wait until the baby is born to do a DNA test, because by then it is about a tenth as costly. (Again, test with both guys. It is not useful to get a "no" for one guy and not have a test with the other guy. I get so many letters from women who did just that, and they all say, "What if the test is --- WRONG?!?!?!" Testing with both guys means one man's "yes" confirms the other man's "no.")
Good luck to you, if it were me, I would let the fact that the baby is a boy strongly indicate to me that the friend and not the ex is the father. I would test after the baby comes, with both men, since important issues of identity and legality hang on the testing. Also, so I would never, ever have to wonder again.
Congratulations on your wonderful upcoming new baby.
An interesting discussion. I don't think you should let the two day difference in estimated date throw your first impression. A late ultrasound is not that accurate. For me the probability is that Aug 30th has a higher chance than Aug 24th, but it's not ironclad.