Hi hun, I have lots of cervix related experience and would like to help if I can.
Your cervix is currently within normal range, but is at the shorter end of normal. It has also shortened significantly in 3wks so I would personally exercise caution despite what the medics are advising.
If everywoman were scanned in pregnancy, there would be a huge variation in cervical length, and it is normal for some gradual shortening as a pregnancy progresses. It is also true that twins exert greater pressure on the cervix, and the extra stretching of your uterus much earlier on also adds to this strain. In women who might otherwise carry a singleton fine with no cervical issues, there is sometimes a 'twin related' cervical incompetence which causes a possible problem around 16 to 24wks. It is one of the reasons why twin pregnancy is considered 'high risk', and the reason your team have been so proactive in checking your cervix with regular scans.
A dynamic cervix is one which constantly shortens and then lengthens again day by day, sometimes even hour by hour! It might well be that yours was 2.9 today, but tomorrow has regained some length and is 3+. This scenario is possible and very common and women in that position can continue like that to term.
It might be that your cervix now remains stable for several weeks, or it could shorten further. In my experience both are equally possible, BUT since you have had an early 'warning' that your cervix is changing, albeit minimally, then I'd personally do as much as possible to give it a helping hand and take the strain off it just to be on the safe side.
It is good news that there is no funneling yet (which is when the cervix opens at the top, but remains closed at the bottom). Shortening usually comes first, then funneling and finally dilation. With an incompetent cervix all this happens without contractions and is purely a mechanical issue as the weight of the uterus gently exerts pressure on the top of the cervix, gradually opening it. Yours is nicely closed and may well stay that way for the duration, but given it has shortened in recent weeks then take it as a sign to rest hun.
You are actually in a great position, because most women do not get to see how their cervixes are holding up. 2.9 could actually be normal for you, and you could just have a short but strong cervix which holds up fine. My feeling is that you should take precautions and rest until you know for certainhowever.
I have Incompetent cervix and gave birth to my second child at 24wks as a result. With the twins I had a cervical stitch placed at 12wks and apart from some funneling at 25wks, made it to my planned section at 38wks. I wish I had known my cervix was 'misbehaving' early in my second pregnancy because I could have atleast done something about it.
Genuine IC is rare, and I don't want to worry you uneccesarily, but now that you have discovered some cervical shortening and are carrying two babies (always more complex), I'd definitely take the opportunity to take things easy chick.
Hope that helps, and please get in touch if you need anything xxx
It can only help you to be safe now and give it a chance to get better, if that's something that can even happen..
That's where I'm at right now. been on strict bedrest for a little over a week due to contractions and I'm doing so much better. 
I don't remember having that at all with my singleton pregnancies, but then they were 16 an 20 years ago so maybe I've just forgotten? 
