Hip bones do not fuse. The tendons and ligaments just become tighter in rabbits, guinea pigs, and some animals making birth more difficult.
We bred one slightly over a year old that we didn't know had never had a litter. The breeder never said anything. She had 2 live and one dead that I think got stuck. She had to have anti inflammatory and pain killers (metacam) because she was so swollen afterward that she just sat in the corner refusing to move, eat, or feed her kits. One dose and she was bouncing around with everybody fed. Her second litter had no problems but was only 1 kit. We are trying for a 3rd since it was such a nice kit.