The second or third season opener had a "pregnancy scare" with the family waiting to see the results of a home pregnancy test, when it turned out she was only late, even explaining how regular she was that she was only late three times before, then glances at the kids.
So they had already teased a potential pregnancy storyline could pop up at any time. When she was with Tom, she tried in vitro, but no success. When she hooked up with Ben, they tried and succeeded with Buck. THIS is what was written into the show: her real life pregnancy.
(She even jokes after the lottery about dumping Dan for some psycho, then dump him for another guy...again, reflecting her real life hich the show was originally loosely based on.)
Same applies to Crystal getting pregnant a second time right after her first with Ed, Natalie had gotten pregnant right as that arc was wrapping up. She opted to stay home with the kids, and take a break from acting, so we didn't see or "visit" Crystal and Ed after that, save a few times.
then Laurie got pregnant while Roseanne and Tom were trying, so they shifted the focus to Jackie's baby when the marriage to Tom had it's meltdown. So they had to write in Fred as the father, because her husband Matt had played her abusive boyfriend Fisher.
The only catches here are Laurie got pregnant as one season as ending, so after the summer, her already lean frame was already showing, and they had to write hers into the show pretty fast.
On Roseanne's, she set it up to play out over the season, ignoring the summer break when she would actually be getting the IVF, but then planned to give birth at a Grateful Dead concert, but unfortunately, Jerry Garcia died right as they were leading into it. So they wrote her delivery into a Hallowe'en/Dead Tribute show, and then named the new kid after him.
Adding in Darlene's was just likely padding out to keep Sara on at that point, and they made mention that she was going into labor too early, likely in part because of some of the complaints over Roseanne's "year long" pregnancy. (Sara wouldn't have her own kid until years later, and by IVF after her first wife had hers.)
In the end, it doesn't really matter, as they reflected changes going on in the actors real lives, and they actually acknowledged the new kids, including them in random scenes after their birth instead of just mentioning "Bev is watching them this week" or whoever didn't get any airtime (Becky/Crystal/etc) like other shows still do.