Would you be planning to have this discussion anyway, or only if the other guy comes up positive as the dad once the baby is born? I'm not trying to be a nuisance here, I am just saying that having this discussion (hard as it is) is worse if it is delayed, since the guy realizes how long the woman knew this was an issue and she kept the fact from him. This is a disclosure that, if you are going to disclose at all, is better done soon and a lot worse if avoided for many months. The guy will see those months of silence (in retrospect) as months of being deluded by you.
What do you intend to say to your husband if DNA testing after the baby is born proves the baby came from the other guy?
I should also mention that although the egg does not last all that long before it must be fertilized or become non-viable, sperm lasts a lot longer. The usual number you hear is 4-6 days in the woman's reproductive tract. Some research suggests longer, other research suggests the sperm is not mighty enough to penetrate the egg after about day 6. This is why I was saying if the ovulation date was the 9th (and there is no guarantee from what you were told that it is, since it was to some extent based on your LMP), you could well have had living sperm from both men in your reproductive tract at the same time. Then it would be just luck of the draw. In any event, if your ovulation was on the 9th that does not rule out your husband.
Well, I was not talking about the cost of the DNA test once the baby is born. They are not too expensive for most people. I was talking about doing a pre-natal test and knowing quite soon, if you have the much larger money it takes to do one. (It's about ten times as expensive as doing a post-natal test.) Mostly the women who do these are those where the question is literally life and death, for them or the baby, and for whom it matters that much to know before the baby is born. I'll send you a pm with some information, I just caution that you only use the top lab in the world for it. (They have relationships with labs all over the world to collect the samples.) A woman wrote in recently who used a cheap lab with a crummy reputation, and she is no better off than she was before the testing, feeling like she cannot know for sure if what the test told her is true. You really don't want that. And for the same reason, you don't want to test with only the one guy. You never want to wonder "What if the test was WRONG?!?!?", which is in fact what happens when there is any ambiguity. You want a positive for one guy and a negative for the other. Those two tests would back each other up and you would not become prey to doubt.
What ovulation means is that your ovary popped an egg out into your Fallopian tube on that day. The egg can live up to 36 hours before it must be fertilized or become non-viable. But assuming that you ovulated at day 14 of your cycle is risky. Some women do ovulate right on the dot when expected. Others do not, even ones who have had a consistent cycle length for a year and a half. Women's ovulation can be triggered by a lot of things (including close proximity to a group of women who cycle at a different time) so you can't just say "My last period was on the 26th and my next period was due on the 23rd, therefore I ovulated on the 9th." The only way you could know that for sure is if you did actually have a period on the 23rd, which of course you did not. (Ovulation leads into the period, the last period does not lead into ovulation.)
Anyway, how dire is the situation? If the baby were to turn out to be from your fling, what would you do? If you need to make a serious decision now if the father was not your husband, you might look into DNA testing now. If you contact Ravgen, ask them about the mechanism of discreet testing, and also about how someone would get their results if they didn't want anything to come to them in the mail. In all cases, stay away from cheapie so-called "labs" that advertise heavily on the Internet. I cannot exaggerate the misery they have caused.
By what method did you get your due date, by ultrasound? If so, do you have any way to know if the doctor based the due date only on the measurements and developmental markers of the embryo and not on your last period? How early did you get the due date, and again, was it from a doc looking at an ultrasound, or did he or she just pull out a little cardboard wheel and base it on the first day of your last period?
You mention that you are "not in a position" to do a DNA test, is that because of the cost? Because you can certainly do a test. Ravgen does discreet testing using someone's toothbrush or a swab from the edge of a drinking glass. It is understandable if the impediment to testing is financial, many women who write in don't do prenatal testing for that reason. But it is not impossible to keep it a secret.
How regular are your 28-day cycles -- if you wrote a bar-chart of them, would it look like a solid block, with no variation at all for months and months? If your periods come every Day 28 and have never failed to come exactly that day for years, and if "pulling out" did not protect you from sperm in the second guy's pre-ejaculatory fluid (which unfortunately it does not), both men could have had living sperm in your reproductive tract at the same time. Just from your report of having 28-day cycles, that could have been the very day you ovulated. But it might be something we could clarify for sure if we knew the exact data about your first ultrasound. Write back. :)