Special Announcement (or so I hope)

My husband's family does a secret Santa every year and this year I pulled my mother-in-law, me and hubby are ttc and I think it would be an awesome surprise to announce a pregnancy along with my gift to her. I am planning on getting her a Mother's necklace with all of her kid's birthstones. If I do end up getting pregnant and can announce by then I'm not sure how to do it. Any thoughts or ideas would be greatly appreciated!