There's really two questions here. One is about post-pregnancy and the other is about couples in long-term relationships. They're completely separate.
I don't think it's any surprise if women look different after pregnancy; there's a whole host of changes that have happened. Note that, in addition to any metabolic changes, there are surely major lifestyle changes occurring. Your life often changes completely after having a baby - lack of sleep being merely one. I would look to these changes first.
As for childbirth and menopuase, I asked my wife, the childbirth specialist, if she knows of any correlation.
She said certainly: childbirth is
dramatically affected by the onset of menopause.
But no, she knows of no correlation in the other direction.