Greenie - fret not my sweet.
There is a higher chance of prem labour with twins, but the majority of these births fall between 34 and 37 wks, which is by no means disasterous.
THe extreme prematurity we are discussing here, and that I have had experience of is for very specific reasons, not necessarily related to twins. My daughter was a singleton and came early due to incompetent cervix. The fact I am now carrying twins makes my IC more of a problem due to the extra weight on it, but for most women this isn't an issue.
It was me who mentioned vaginal discharge because when my daughter came at 24wks I didn't go into labour, but did have lots of milky discharge. It was my only symptom, and one typical of an incompetent cervix which is dilating. This has made me paranoid about discharge in this pregnancy. However, with my first, full-term baby I ironically had more of the stuff to the point it would run down my legs (sorry) and I thought nothing of it. My son was born at term and healthy. Again, discharge is usually a normal, healthy part of pregnancy and it increases naturally as the pregnancy progresses - it is only if there is an abrupt change in its amount that you should be checked out. I have had it in this pregnancy, and it has meant absoultely nothing, but naturally with my previous experience I can't help but worry.
Premature rupture of membranes and preterm labour can happen in any kind of pregnancy for a whole host of reasons, but is still rare. Twins are usually born earlier than singletons, but still mainly after the 34wk mark. Remember the average for twins delivery is 37wks anyway - which is considered a good gestation.
Relax and enjoy your pregnancy. Us ladies with complications share details because it helps us to get thru after previous/current traumatic experiences, but we are by no means the norm