Like she is.

"Fuck me like that, Kako," you tell her. She is happy to oblige, and her tits and balls bounce along with her. You rest your hands on her giant ass cheeks, and enjoy the feeling of her muscles working for you. Kako picks up the pace a bit as Mrs. Barstone cums again, her pussy contracting with a tight grip on Kako's cock. You feel her balls tremble, and know Mrs. Barstone is getting another gallon of cum in her womb. Having had her belly expand with all those kids, you know she will take it like a pro. You rub Kako's balls, and pinch her nipples, trying to get her to cum more for her pregnant fucktoy. As Mrs. Barstone convulses, you feel a few droplets hit you as her tits spray milk in your direction. You just lay back and join them in cumming, pumping your load into Kako's fine ass. Even though you are the last to start, you are the first to finish. You cannot compete with the supernatural orgasms of your fuckmates. Mrs. Barstone looks like she is going to come down next. She opens her eyes, and looks at her situation. She definitely grew again, and you have managed to keep Kako's orgasm going, so she is still growing. Her belly has overflowed her sides, with a big soft roll hanging down on each side. Her breasts are being pushed upwards, and on the bottom it is hanging lower than when she came in, covering half her pussy already, and still going. You get harder off of this, and pump up at Kako's now well-lubed ass, and say, "Kako, look at Mrs. Barstone." She does, and groans, her orgasm continuing due to your assfucking and Mrs. Barstone's ridiculously distended pregnant belly. She manages to cum for another full minute because of your stimulation. She eventually relaxes though, laying her head on her tits which are laying on your chest. You continue to fuck her ass, while you enjoy knowing something Kako doesn't. Her cock is trapped inside Mrs. Barstone, who's belly looks like that of a 500 pound woman, flowing over her legs and sides, and down to her asshole, trapping the flexible cock inside of her. But you know that isn't fat, but your woman's semen. Mrs. Barstone rubs her belly, "Wow, this is the biggest I've ever been. I love the feeling."
Kako looks up at that, and you feel her balls twitch again. You reach down and start rubbing her cock quickly. She knows theres nothing she can do. You have her impaled up the ass, and a good part of her cock is trapped under the cum-swollen belly of Mrs. Barstone and inside her giant cunt. "How can you want more?" Kako asks her, "Won't it make you explode or hurt the baby?"
Mrs. Barstone smiles. "After you have been through 8 births, you know your limits. This barely feels stretched at all. Not like my first. When it starts feeling stretched, I will have another 9 months pregnancy worth to go before my body will have to really work. Besides," she gives her cunt a squeeze, "you aren't getting out until I'm done with you!" Kako begins ejecting another load into her belly.