Bernie says on screen she’s pregnant. Sex is manifestly implied. The date isn’t. The sex of the child isn’t. Heck, if this weren’t a sitcom, the name of the father wouldn’t be, but it is a sitcom of the type it is and therefore the father is Howard.
Leonard and Penny have had sex lots of times. By the reasoning some people have used, Penny could have had five abortions by now. Or several miscarriages. Or could have cheated, or been part of a threeway, or been spied on by hidden cameras with footage sold to Serial Ape-ist fans. (The existence of the movie wasn’t revealed until season seven, although she retroactively made the movie before she met the guys.)
That’s the core of on screen lives. The missing parts don’t exist, unless and until they are put on screen in some form. All your complex fanwanking can be undone in an instant, even if only by an actor’s throwaway ad lib that the showrunner decides to leave in. That’s the number one rule (yep, I went there) of sitcoms.
Rule zero is that if the show violates your assumptions too many times in too many ways too many yous don’t like, it will go off the air. But then you get to spend all your time on missing parts, you lucky fanwanker you.