The ladies are right - relaxin affects the ligaments that connect the pelvic bones in the back - and one of the main structural differences between men and women is this ligament.
Narrow hips don't always indicate they won't spread far enough even as wide hips indicate they will. When pregnant, I was deemed "big enough", but was unable to deliver my two normally at all (once C-section, one forcep delivery (they waited too long to do a c-section)).
My hips aren't any bigger now than they were before I had children (and I DID have the second one "naturally" - forced as it was). My hips are pretty much the same size at the same bf% as they were before I had children.
So it doesn't mean that hips will or will not go back to their normal size. The relaxin productions shuts down during the months after pregnancy and I suppose it depends on what one is doing when production stops. I know it isn't immediate, because I got pregnant four months after my first one, and ended up putting my back out something fierce over it, and my doctor blamed it on not having time for relaxin levels to normalize between pregnancies.
Relaxin is also produced cyclically during the menstrual cycle at much lower levels, starting about 5 days after ovulation and peaking about a week before menstruation, then dropping.
I tend to get agonizing lower back spasms a couple of days before my periods (not associated with menstrual cramps). Many years ago (before I knew I had FMS), my physical therapist told me that the premenstrual drop in relaxin was causing that pelvic ligament to "seize up" and the spasms were twisting the whole pelvic structure out of alignment.
People with FMS are theorized to be sadly deficient in relaxin - which may be why I couldn't deliver those babies without all the machinery (it didn't help that they were both 9-pounders, either). However I have seen "normal" women pop back to their previous narrow hips, and seen some develop very wide hips after a couple of pregnancies. Relaxin levels during pregnancy are about 10 times higher than those associated with your menstrual cycle.
And if you think about it - it may be not just having children, but the cyclic loosening and stretching of this ligament over years and years, may be what causes the "middle aged spread".
BTW - this is why "juniors" clothes (designed for teenagers and younger women) are proportioned with a smaller waist to hip ratio than "misses" sizes. The waist size in comparable sizes (say a 4/6 misses and a 5/7 juniors) will be the same - but the hips in the misses size will be more generous.
Don't mind me - I'm babbling.
Fawn