How do I get out of going?!

Some of my family wants me to go to a concert, but I'm pregnant and have no desire to go, but I don't want to tell them just yet that I'm pregnant but I need to get out of going! Help!! I need a good excuse! Thanks!

Edit: my stepmom wants to know like tomorrow because she's buying the tickets.

Edit: the concert isn't until March 22nd, which is a Sunday, I think.

*****THANK YOU EVERYONE for your suggestions!! That week is my kids' spring break, I just realized, so I'm going to say we have plans so I'll be out of town! Haha saved by the older kids' Spring Break!*****