It's hard to say unless you get a sonogram and have the baby measured. Even then it can be two weeks off. I'd say it doesn't really matter at this point, since you're already pregnant!
You have to have at LEAST ten days between ovulating and your intended period for the egg to have enough time to implant and begin giving off preg. hormones.
But you could've ovulated later, thus having a later period (which you wouldn't have had because you're pregnant). So it's just anyone's guess.
Well, if you're 100% certain your period was due on the 19th or 20th, then conception would have had to happen before the 15th. BUT, your ovulation could have been delayed a few days that cycle, and if so, you could have conceived on the 15th. In that case, your period would have been due between the 27 and the 31 (or so) instead.
Basically, ovulation takes place about 12 to 16 days before your period, and the egg can only be fertilized for 12-24 hours once it has been released.
Hope that helps!