We don't have the friendliest relationship with DH's ex, nor does she try to keep things civil for her daughter's sake. However, the first time I was pregnant, we pretty much just said screw it, and told DSD right away, as we were so excited. She did tell her mom, who was shockingly very supportive about it. Sadly, that first pregnancy only lasted about 8 weeks, until we found out it was a blighted ovum. We told DSD on one of our nights that we were losing the baby, and she had a school program a few nights later, when we still had her. Her mom approached me there, offered her congratulations, and asked very nicely how I was feeling. So I then got to tell DSD's mom that I was miscarrying. That was fun.
We asked DSD after that pregnancy if she wanted to be told so early, or if she wanted us to wait to tell her until we had SEEN a baby and the odds of miscarriage were a little lower. She chose to have us tell her. So when I got pregnant again in June, we told her, and asked that she keep it to herself for a while. Thankfully, she was able to do this, so when I lost the baby a few weeks later, we were able to mourn in privacy.
I do think from DSD's mom's reaction to the first pregnancy, she will choose to be supportive if I am ever able to maintain a pregnancy. I also think that at DSD's age, she would have felt very left out had we chosen not to tell her that I was pregnant right away. When we told her the second time, she asked how long we'd known, and was very happy that we'd only found out a few days before. Autumngrey, I think DSD would have been horrified, and felt very, very left out, if a month passed between a sibling being born, and her meeting it. As we have 50/50 custody, this wouldn't happen for us, thankfully. I also don't think DSD's mother would ever be so hurtful as to let that happen, so our relationship with DH's ex must be a little better than yours. I'm sorry for that.