It's not possible really to know in advance when you'll go into labour, though you may have some intuition about it. But all those objective signs, like dilation and thinning of the cervix, no really don't mean anything. You can be 4cm dilated for weeks or you can go from nothing to having a baby in about an hour. I think the average spontaneous labour for a first baby is about 9 hours of active labour. So in theory, yes, he would most likely be able to make it back for baby's actual arrival, but would you have someone else who could be there to support you in his absence and would you want to have someone else? Also, how long is the conference? If it's just a day, it's unlikely, especially if you're worried about it that you would suddenly go into labour on the one day he is away, but if it's like for a week, then the chances are obviously greater.
My first was born at 37+5 naturally, so I didn't even make it until 38 weeks. It was 11.5 hours from start to finish. I had absolutely no signs at all that I was about to have a baby! Though my contractions were about every 5-6 minutes from the start, the first roughly 4 hours I was fine and actually didn't really want anyone around me and mostly just stayed up in our bedroom alone and on my own while my husband cleaned and got things organised downstairs. I had a home birth, so slightly different as we didn't actually need to get up and go anywhere, but definitely from hour 4 until she was born, so for the next roughly 8 hours I did want him there, at least nearby, even if he didn't need to do much. I would say there's no harm in him saying he'll go, assuming it isn't a big expense that he can't re-coup if he cancels last minute, but with the understanding that it's potentially 50-50 that you might need him there when it comes to the day. As long as he's happy to pull out and that won't cause any drama with his work, then yes, I'd say plan for it, but re-assess as it gets closer.
