When they told the Doctor, he was very quiet at first. Barbara thought they ought to have expected it. After all, they'd just informed him that in another seven months or so they'd have an extra person aboard the TARDIS, albeit a very small one.
"Is everything quite all right?" she asked, hesitantly.
"Hmm? Oh. Oh yes. I was just thinking of how Susan would have loved to be here for this. She always hoped that you would have kittens someday. She'd even thought up names for them." He sounded so wistful, she almost felt bad about correcting him.
"Human children are called babies, Doctor," she said, gently. "Not kittens."
"Are they? Hmm. Well, you do seem to have taken your time about it. I'd have thought the two of you would have started to reproduce ages ago. After all, I was very careful to find dear Susan a breeding pair."
It was at about this point that Ian started shouting and Barbara started wishing they'd never mentioned it in the first place.