MIL is telling everyone my unborn child's name

So my husband and I found out a few weeks ago that we're having a boy! Yay!

We had already picked out a definite name for a boy and girl, so we knew right away what his name was going to be. We shared the name ONLY with our immediate family and told them to keep it to themselves as we wanted to announce it ourselves after his birth. My in laws were not crazy about the name, of course. But it didn't bother me.

So fast forward to tonight, we're having dinner together and spending some time in their home just hanging out. I overhear my mother in law speaking to someone on the phone (sounds like one of her sisters) and low and behold, she repeats my son's name not once, but three times, then giggles. I am furious. I am upset. I am close to tears.

I want to keep the name secret until his birth to avoid any unnecessary comments and criticism, and personalized gifts with his name on it before he arrives.

His name was special, anticipated news I wished to share myself as the mother of the child - my first and possibly only child - and now I feel so robbed of that. Who knows who else she has told?! I am so upset.

Should I confront her about it? Should I tell my husband and have him confront her?