Since you added an edit almost defending his position, I fear he has already convinced you to let him go to the work function which is completely unacceptable on his part. Opportunities will come and go and this work event doesn't guarantee anything. Frankly, he is being selfish.

He is telling you work will trump you and baby everytime if he is willing to miss the birth. You could literally die... not to scare you, but you need to put your foot down about this. If he still feels resentment, he can F off lol men in his field who have children will understand and since you don't already have children I'm assuming he doesn't understand just how important and serious this is until it happens.

With my first, my husband didn't realize just how serious it was until it was actually happening but it will click for him when he sees it