A mother brings her 17-year old daughter to a doctor to find out why she's been feeling ill for almost 6 months straight.
"Her mood changes all the time, and she gets weird cravings," her mother tells the doctor. "She's also gained weight faster than ever before."
The Doctor looks at her and says, "I'm going to be honest with you, Mrs. Johnson. In my opinion, I'd say that your daughter is about 6 months pregnant by the looks of things and by the symptoms that you've described."
The mother becomes angry and snaps, "Impossible! She doesn't have a boyfriend! In fact, she's never been with a man! Have you, Sandra?!"
"No, mother. I never have," Sandra, the girl, replies.
The Doctor nods to himself, stands up, walks over to the window and stares up at the sky. After about a minute, the confused mother asks him what he's doing.
The Doctor replies, "The last time this happened, a star appeared in the sky and three wise men arrived for the big event, so I'll be damned if I miss it a second time!"