Have you ever seen that movie with hugh grant and robin williams (Robin williams plays some Russian doctor) and at the consult the doctor tells the couple "ok you concieved this date" and the guy gets all mad and says "its not mine, I was on a business trip!" and then the woman gets hysterical and crys (I recommend watching it, its funny and you can probably relate to the pregnancy symptoms). The thing is unless you were abducted by alieans in the middle of the night who inseminated you, theres no way you could have conceieved any time other than within 1-5 days from the night you were together. So that should solve your problem. The other thing is that most babies arrive early or later than their due dates, hardly any arrive on the day. Also full term is considered anything from 37-42 weeks so if you even narrow it down to within a week or so of knowing you should be good.