Why is M/C common within 19-22wk gestation babies???

[supertwinpower]

Hello
I've been reading some of the post and I have noticed many moms went into preterm labour between 19-22wks regardless of maternal age. I also lost my son at 19wks 4yrs ago but God managed to let me keep his twin sisters (triplet gestation). I was trying to find info online as to why this is so common....could it be that the process is done artificially, that our bodies reject the babies after awhile??? I don't know.....if anyone knows of a site or have info on this subject please send it this way....my husband and i want to have more children but are really worried about the risks....since the first time was an extremely bumpy ride...
sandra