I'm not a fan of it. I really can't stand roleplays where pregnancy is the key theme, because it's just an idea that holds absolutely no appeal to me. I don't want to be fathering kids any time soon, so I don't want to roleplay it either.
As for characters being coincidentally pregnant in a roleplay... well, sometimes it works and sometimes it doesn't. If two characters in the party fall for each other and then have a night of passion that leads to pregnancy, then I don't have too much of a problem with it. It's going to be early stage pregnancy, so it won't overly affect the woman's abilities, and it's developed naturally as part of the roleplay.
What I'm really opposed to, however, is when someone starts up a roleplay and the gist is that we need six adventurers to save the world in some kind of a medieval fantasy epic, and a six-month pregnant human character signs up. That's not right. That character will just get in the way, and slow things down and their entire character is usually defined by their being pregnant, so if, all of a sudden, they were no longer pregnant, that character would have no other personality traits. I know some people would make a pregnant mage (or other character) and actually play them as a fully three-dimensional character with a real personality and skills, rather than just a big pregnant blob, but it's been a rarity in my experience so far, so it's something I just steer clear of when possible.