Newly weds

Tracy, the newly-wed wife said to her husband, Bill, when he returned from work, "I have great news for you. Pretty soon we're going to be three in this house instead of two."

Bill started glowing with happiness and kissing his wife said, "Oh darling, I'm the happiest man in the world."

But then Tracy said, "I'm glad that you feel this way

Spoiler:

because tomorrow morning my mother moves in with us."