Hope you get the results you want. At least you will have the knowledge to make a decision; for us the uncertainty was the worst phase.
FWIW we had a far higher than 1 in 2 chance (>99% in fact) due to a 14mm nuchal fold measurement (well, 4+mm fold with a 9+mm fluid-filled cyst coming out from the tip of the ear to below the shoulder).
I'd dithered about amnio because of the risk of miscarriage (somewhat ridiculously, with hindsight, as the consultants and fetal medicine MW were trying to prepare me for the probability of miscarrying before the amnio date). However, the stats quoted are way out of date anyway, around 20 years old, and the experience of the person doing it also matters. In our case the consultant said that he had done several hundred amnios and not lost a single one to miscarriage as a result of the amnio - but some had miscarried due to the chromosomal abnormality. E.g. If it turns out to be Edwards/Patau, miscarriage is likely anyway.
We too had made the decision that if the condition was incompatible with life, we couldn't go through with the pregnancy as it wouldn't be fair on our first child - and that it would be easier on both of us, particularly me, if it was done at the earliest stage possible. I think there are very few people who would argue with that decision - anyone who's been in the position understands that each couple needs to do what's best for them as a family, physically and emotionally. For some people, continuing with the pregnancy until it's natural conclusion is important, but for others the impact on existing children takes priority over all else. For the sake of your children, I think it's really important to understand what you're dealing with as early as possible too.
On the other hand, if it turns out not to be a condition incompatible with life, but a condition which may involve heart defects, etc, the amnio results will at least alert the doctors and you will be sent for further detailed scans. If you went ahead you'd have appropriate care on standby for the birth and be giving your baby the best chance.
Don't give up hope until there's none left, though. Despite the measurements and the less than 1% chance of viability, our amnio came back clear and the cystic hygroma spontaneously resolved over the next few weeks. All cardiac and abnormality scans came back clear too, and our miracle son is now 2.5. I'm all too aware that good news stories are few and far between (three previous miscarriages...) but, you know, just sometimes things work out so much better than they first appear. I wish you the best of luck.