Loula I'd have the scan at 16weeks and then the usual morphology scan. They will be able to compare the two scans (although not always accurate as the cervix does change lengths especially as pregnancy porgresses) if in the event the cervix was shortening at your 16week scan chances are its still long enough that if a suture was to be placed the risk wouldn't be to great to put the suture in. In my experience as a midwife the suture usually goes in around the 15-16weeks (can be earlier or much later) with women who have a history of IC. The longer it was left the higher the chance that the cervix is already dilating or to short making the risk much greater.
In saying this I am sure it wouldn't be neccessary but maybe it will give you some piece of mind and another chance to see your bub. A scan is not invasive either.