Actually, the cross sections I've seen follows this method for mammalian pregnancy; the basis being that if the babies were just floating in the bean paste, they'd be digested. See also, the Yun-Yah castration kit, and Yukkuri Bonsai.
Yukkuri Bonsai was one of the ones I was thinking of, actually. It doesn't recommend filling the mamumamu with bean paste, but sealing it up. The text also seems to indicate that the baby being reabsorbed doesn't start unless it floats in the bean paste for significantly longer than a normal pregnancy. post #12026 , third panel.
..Why am I arguing *for* what I like less? This is silly. INTERNAL STALKS IT IS.
Also, if they were just floating around inside, there's be more risk of bean paste being expelled during birth, especially with the velocity some birthings seem to achieve.