Hi S., I just had twin boys at 38 weeks a month ago. I was 3 cm dilated and having regular contractions for three weeks and I still needed to be induced at 38 weeks (my doctor didn't want me to go past that and good thing as my boys both weighed in at 7 lbs when I delivered!). I was almost 4 cms dilated and 80% effaced when I went into the hospital to be induced and it still took major amounts of pitocin and breaking my water to get things moving. I was in labour for 11 hours in total so those boys were just not coming out even though Baby A was head down and engaged for three weeks, too! If you haven't had contractions, it's probably a good sign. What does your doctor say? Obviously for me it was totally possible. I wasn't checked before 35 weeks, but if I was 3 cm dilated by then, I'm pretty sure I was dilated before then, too! As for keeping them in, remember to rest, rest, rest. That's probably the only thing you can do to not bring on labour. I was told to put my feet up at 35 weeks so I would make it to 36 weeks and I went to 38. I found lifting my son or any other heavy lifting and lots of activity brought on contractions so I hired a babysitter for a week and literally put my feet up. My husband did all the housework, etc., and then my parents arrived to help when I was 36 weeks and I did nothing until the birth...
Anyway, good luck with the labour, etc. If you ever need to talk about having twins or need support, don't hesitate to contact me (____@____.com). I'm fairly new to it still, obviously, but it's been working out well so far... Biggest problem is my two and a half year old! And I do know a few other people with twins.