In Chip Off The Old Clark, when Klein is taking DNA's samples from Superman for a paternity test, we get:

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DR. KLEIN (CONT'D)
The results will be quite definitive... Incontrovertible, really.

SUPERMAN
Great, Doc. And you'll have those results when?

DR. KLEIN
Well, based on what I suspect about the complexity of your cell structure.... Four to five months. A year, tops...

SUPERMAN
What?!
In Shadow of a Doubt, we get:

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DR. KLEIN
Aha. Obviously, your physical and chemical makeup is quite a bit different than your lady friend's and that could be a problem. You should proceed with extreme caution at any attempt at procreation. There's no telling what could happen.
and

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DR. KLEIN
What? Well... Here's the thing. While in many ways you resemble an earth human, your biology is clearly different. You couldn't donate blood, for instance.
Now, the script we have for SoaD seems to have several versions of that scene, so I'm not sure which is the one that actually aired. But I am fairly sure that some comment was made along these lines in that particular episode.

Running out of computer time here, but I'd also suggest checking out the scripts/episodes of Voice From The Past and The Family Hour - which will probably have similar dialogue.

LabRat smile


Athos: If you'd told us what you were doing, we might have been able to plan this properly.
Aramis: Yes, sorry.
Athos: No, no, by all means, let's keep things suicidal.

The Musketeers