We found out with Molly. Our situation was that we REALLY wanted to have a girl, and if we waited until the baby was born, and it was a boy, then we'd be disappointed. BUT....if we found out halfway through then we'd have a chance to get used to the idea. We had amnio to check on everything fully, and there was also the added bonus of knowing for 100% sure what the sex would be. We'll be doing it again this time too.