I have just heard he left in October and she would have become pregnant in September or October. Probably late September/Early October, IF we assume that 4 months and 5 months pregnant are EXACT numbers.
If she was actually 4 months and 2 weeks pregnant when she disappeared, then conception would be earlier. We just don't have enough information. She did appear pregnant in the pictures from Hawaii... at only 4 months...
A week difference by itself, is not enough to be certain who is the father. Would one week be enough for you to say, this baby isn't my husbands... it is my boyfriend's? (Obviously you wouldn't be likely to be in that situation... but it likely wouldn't be enough of a time difference for you to decide that less than 4 months into the pregnancy.)
An ultrasound is also an estimate... a baby can be measuring more than a week off... that wouldn't be conclusive.
So Bethany told her family for some reason that she was sure it was the boyfriend's baby. I don't think she would have done that if it was a one week difference... that's a pretty bold statement to make, especially if she told her husband too.
I think we will find that the husband left earlier than we think, for training or something... or that he wasn't sleeping with her or was using protection. Something that would make the length of time more than one week.
I could be wrong though... maybe she just "knew' it was the boyfriend's baby...