When out in public, Twilight and her partners made sure to keep her bump hidden with a disguise spell, and for awhile this worked beautifully. Then the magic surges began; the dark magic surges being troublesome in particular. It was hard to then explain Twilight's sudden habit of never leaving the castle to both her friends and citizens of Ponyville, but Twilight, Sunset, and Flash as well as the foals managed to keep suspicion from growing too high. It was hardest to explain to Spike to why Twilight couldn't see him (Twilight feels horrible for not telling him for seven months before caving and allowing him in and confiding in him).
She stayed in the castle for the remnants of her pregnancy, before she tries one last time to go outside, nine months in. She'd only gone out due to the twins' constant begging-she never can say no to them as none of the parents could-to see a traveling show. Flash and Sunset were away on royal business (covering for her, again), and the twins had been to well behaved during the whole fiasco, Twilight thought she could handle herself.
Well, she wasn't known for being right all the time, now was she?
ahhh sorrryyyyy, i just have had this story in my head for mONTHS (flurry's involvement is a recent addition of course). soOO i'mjust cranking out work.
have a pregnant, dark magic-using twilight about to slap some pony's ass for being rude to her babies. and then immediately after going into labor. whoops.