O.k.., here's where the book learning comes in I guess.
Several weeks back, an instructor brought in the bones of a cow (rear-pelvic area), assembled in place. He brought up a good point -the discussion was about EPD's and picking bulls- that the calf only had the space between the bones to come out. If it were too big for that space, too bad. Sure the calf could be pulled, or a c section done, but wouldn't we all rather just go out and see a nice healthy calf sucking on moma?
I say go with what got you there. If you have minerals, and sufficient feed(hay), why chance a problem with a increase in protein in the last portion of the cow carrying the calf? This is assuming your cows are in good condition already.