It depends entirely on the woman. We've all heard of the girls who have successfully hid thier pregnancy until the birth....... A pregnancy test is going to be a far better indicator
Oh yeah I can just see it now.
Man: "Um, honey do you mind taking a pregnacy test."
Woman: "Huh? Why?"
Man: "Well, I've noticed that lately you've just seemed a little on the heafty side and then there's the mood swings--"
Woman: "What!!! Why you *#^$%!!! Are you calling me fat?"
Man: "No, no honey! That's not it at all. I'm sure you'll be just fine after some sit up's. I'll even do them with you. "
Any man who recovers from that one deserves a salute.