Like you said, the pelvic bones fuse after a year. Once the female gives birth, then the bones are opened up and stay opened up.
This is true in most animals and humans! That's why your body is never the same after you give birth.
You can weigh the same, but your hips are much wider.
Please don't breed if you don't understand this.
My newest girl AnRa born 12-10-19, came home 2-13-20
RIP dearest Rana 4-19-15 --12-12-19, I will miss you forever
RIP my dear Nara 7/2/10---1/6/15. Mommy will always love you.