We're not sharing the gender until the birth for a couple of reasons. One, because what if they're wrong? They're never 100% sure unless they do a genetic test, and I've heard enough horror stories about parents getting ready for a certain gender, buying all pink or blue shit, and then getting a big old surprise when the baby comes out. We would like to avoid that inconvenience.
Second, we just decided that we want to keep some things to ourselves. We decided to keep the name and the gender a secret just so there's something to look forward to (other than the actual birth of the baby).
But really, I just get off on being withholding.