[beyond the sea]

"I'm so ugly that when I was born my mother took one look at me and demanded an abortion." A dead, stony silence immediately ensued.

The audience was silent for the reason that you suggest, but also because the joke was poorly constructed and NOT funny (no matter the feelings about abortion).

[mamelukesabre]

No.

This was the seventies. People didn't do jokes about crap like that back then. Not on the Carson show anyway.