"Momma’s Growth" joke

When I was six months pregnant with my third child, my three-year-old came into the room when I was just getting ready to get into the shower.
She said, “Mommy, you are getting fat! ”

I replied, “Yes, honey, remember Mommy has a baby growing in her tummy. ”

“I know, ” she replied, “but what’s growing in your butt? ”