Also, I might be a bit naïve (but I haven't given birth so will be), but surely multiple births is no difference to one birth? Does it really get looser and looser? The babies don't get any bigger. The vagina is designed to do what it does..
Not a doctor but I think the main problem would be the constant, back to back pregnancies. Just like it’s not good for your body to be weightlifting everyday, it’s not good for your pelvic floor to be constantly having to support the weight of pregnancies (which can be very heavy considering the baby, placenta, amniotic fluid etc) without enough of a break to recover. The muscles will get fatigued and weaken, meaning the woman will be less able to control her bladder/sometimes even bowel, and more at risk of a prolapse, because the muscles have been overused and aren’t strong enough to hold everything in anymore
Pelvic floor exercises can really help but I think even they have their limits (the limit probably being 20 kids, sure when would you even find time to do them?)
I apologise if I offended anyone with my comment which was influenced by my own vagina gazing
and definitely recommend pushing for help if you have pelvic floor issues (after pregnancy or otherwise), my physiotherapists/gynaecologists have been amazing