An elderly man tells the Docto…

An elderly man tells the Doctor he is planning on marrying a women of 30, and would he have any suggestions.
Yes, says the Doctor I would advise you to take in a boarder.
A year later at his 80th year check-up, the Doctor asks how everything is going. He says fine his wife is pregnant.
The Doctor remarks, so you took my advise and took in a boarder ?
Yes I did, is the reply, and she’s pregnant also…..