"It's better if your son tells you about the news."
Don't shirk your responsibility to have an open, honest and caring relationship with your mother-in-law. And don't use your husband's relationship with his parent as an excuse to do so. Even if the news is better coming from your husband, say "I will let Henry talk to you about this, because I don't want to steal his thunder! But I'm sure you'll be amazed/excited/in stitches."
In short, don't refer to your spouse as "your son/your daughter" to her - it's aggressive and rude. She named her child for a reason. Besides, it's not as if you didn't marry that person.