Twins very RARELY carry to "full term" full term for twins is earlier than a singleton pregnancy. If you have a very small frame it would be unlikely that you would carry to 37 weeks(term for a singleton) twins get crowded sooner than single pregnancies do. Thus they are more likely to be born "early" (earlier than the 37-40 weeks). That being said it IS possible to carry past 37. My mom carried my twin sisters until 38 weeks! But they were extremely big babies (6 and 7 lb twins!) Good luck
if your carrying twins they usually like them to be delivered at 37 wks if they dont come before then :)
I think they consider 32 or 36 weeks full term with twins. Your doctor/midwife will be able to give you all the info :)) Congratulations by the way and goodluck!!
Hardly any twins go full term, despite mom's size. Communicate often with your doctor and he/she will do their best to help you stay pregnant as long as possible. Good luck!