Welcome amy bella haribo moonlight and anyone else I have missed. 
to see so many new faces here but you have come to the right place for virtual handholding, hugs, and even giggles. We have all been through so much, more than anyone deserves, but yet there is still humour here.Its kept me sane.
moonlight my first baby also had trisomy 18, Edwards syndrome. 'Luckily' for me it was picked up at the 12 week scan. I can't imagine how you felt getting to 20weeks to receive that news.
I'll never forget that scan. I was the last appointment of the day and they were running late. When I first got in there a trainee sonographer started but almost immediately passed over to the experienced one (I realised later why). I was so thrilled to see a proper baby with a heartbeat, a missed miscarriage had been my biggest fear. But I wasn't as thrilled as I thought I would be, I felt quite detached, like an out of body experience. I think I knew something was wrong. The sonographer was just so so quiet. After ages,about 30 minutes, she said 'that's done' and I had a rush of relief (surely if something was wrong she would have told me). Then she turned and looked at me and said 'there's something wrong with the baby'. And told me all the wrong things she'd seen - nuchal fold of 9mm, leaking heart, no stomach. It was the worst moment of my entire life. I was terrified. We elected to have a termination after the trisomy 18 was confirmed. It was such a difficult thing to do but it wasn't a difficult decision. We spared him for being born only to suffer for a few days. You did the same for your baby. It's the most loving thing any mother could have done.
This happened in January, in April I for pregnant again, which ended in a MMC in June. Like you I feel so sorrowful and angry that it seems to be so straightforward for other people. Why did we have to be the unlucky ones? The first time the midwives assured me we had just been very unlucky, now I know that there's no law that means it can't happen again, that we've been through enough.
BUT the odds are very much in your favour to have a happy healthy pregnancy next time. As I'm sure you know, Trisomy 18 is a random mutation, it's not inherited. Miscarriages and chemical pregancies are sadly much more common but a healthy baby is still the most likely outcome the next time you get pregnant.
I always think the most awful part of what we are going through is that it is grief, layered with fear (will we ever get pregnant again, will that ending miscarriage). But all of us are brave, one day at a time. Better, brighter, happier times are ahead, but you will never forget the babies that you lost.
I am south east london so not too close to surrey. It's nappy valley round here though so I understand how it feels to be surrounded by babies.
P.s next time I am going to have the harmony test at the Fetal Medical centre. It's ££ but it means at 12 weeks you know if baby has any of the major trisomies without the need for invasive testing.
Sorry that turned into a masive essay! It feels good to write it down tho sometimes, doesn't it?
Hugs to all xx