(This apparently isn't an issue for the also pregnant Cobie Smulders, whom they must have thought was going to be showing less when writing these episodes because flowing tops be damned, she's also still pretty obviously pregnant, even she had Ted's assistant on top of her.)
So the offending joke Barney told was "What's the difference between peanut butter and jam?"
A possible answer, according to a quick Google search is below and censored for those with delicate sensibilities...which I am actually doing as another level of humor because seriously, what are you doing reading this site then... Highlight it if you need to read it:
"I can't peanut butter my dick up your ass
Which I actually find pretty funny.
Update: If you came here on a Google search to find the answer to this joke, as many people do, please support my site by clicking on an ad below that may interest you. Post continues below.
See, there's lots of differences between peanut butter and jam, but... okay, despite what PunkAndy may believe, explaining jokes DOES NOT make them funnier, and this is also true when explaining how jokes can be improved.