I'm 6 weeks pregnant with my first. I did lots of scouring of the internet on this before trying. I'm continuing as usually. I just adjust my pace slightly, keeping a pace where I can maintain a conversation, so going roughly 85%. I actually won our in house comp in the scaled division (I'm better at the gymnastics than the weight lifting) the day before I got my BFP.
I just completed Angie this morning! I did tell my coached immediately, and they are still the only ones who know. We had another member continue right up to the week of birth. Your coaches will be able to help guide you with modification, but no scaling or alternatives are needed until the end of the first trimester. Then no more situps and eventually you'll have to adjust for the bump. I plan to continue right up until birth, assuming my body lets me.
There are several blogs about. I've even found another box near me who have new mum membership that provide childcare during your rehab sessions. Whilst I love my box and would continue there once 100% after birth, I may join the other box for their mum rehab classes as soon as I'm cleared after birth.
Plenty of people still doing it. I am continuing to go 4 days a week at the moment. It gives me energy. I've only been doing it for 5 months, but that is long enough to continue whilst pregnant. If I have to reduce to 3 days, I will. I'll adjust when my body tells me to. Listening to your body is the most important thing. There is one crossfit mum who said she had to stop in her 2nd trimester because her body just wouldn't allow her to continue, and this was after continuing with crossfit throughout her previous pregnancy.
Hope this helps!