Hi Lou,
Congrats on your pregnancy and sorry you've had some stresses. All of your measurements look good to me so far but clearly if it's a case of running out of room it's not that straightforward!
If you read back over this thread you'll see we've all had very different experiences, it doesn't seem like there's definitive guidance on what's best to do unfortunately. To give you my answers to your questions:
I had a spinal for the stitch. I think this is the 'least' anaesthetic they offer, if you see what I mean. Like Kelly says, it wasn't pleasant but was quick and straightforward.
I don't think there's a 'latest' for the stitch, I was told by my prem clinic that they worry most up to 27 weeks, after that it's less of a concern, so maybe if you get past 27 weeks they'll find another way to manage it? From what I've been told it's better to have a preventative stitch than to wait until there's a problem and then put one in - that's when most of the infections and complications seem to occur (although still a good chance of it being fine).
At my hospital they will always recommend a stitch if your cervix is shorter than 2.5cm, but I know elsewhere it's not guaranteed and some hospitals treat in different ways. For example, I found out this week that my stitch is nylon, which is different to most other hospitals. Also I've not been given progesterone or antibiotics...nothing, whereas our sister hospital gives antibiotics as standard after the stitch has gone in and a lot of people on here seem to have been prescribed progesterone too!
In terms of the risk, I was told the 'official' risk is 1:100, however the doctor told me confidently that she'd never had any problems with putting a preventative stitch in and she does about one a week (on average). If your doctor isn't confident then could you look at getting it done at another hospital? I don't know where you are but I know Queen Charlotte's in London specialises in this stuff, and I think Guy's and Thomas' is also very good.
I'm not 34 weeks and all seems fine so far, life has gone on pretty much as normal and I'm now counting down the last 3 weeks until the stitch comes out. I'll never know if I needed it or not, but very pleased to have carried a healthy baby this far.
Good luck and keep us updated!
Piz