Kind of echoing mostly what everyone else has said!
I totally get where you're coming from. I know myself that when I have a plan in my head it just has to happen like that and I don't cope very well with change. I'd also know I'd be particularly upset if my CBM couldn't make some of my wedding etc as I've gone through this!
On the other hand, having a baby is a massive deal, much more so than your wedding (maybe not to you of course but to her!) I think you just need to support her in this so not to risk losing from what sounds a VERY CLOSE friend and wait and see what happens.
There are lots of possibilities.
If she has the baby BEFORE your wedding. I would keep her as your CBM. I think she's definitely be able to be there for you on the day as presumably her partner would be around to take care of the baby. As for the hen night, if you still want to go abroad, DO IT, she can't dictate that, its YOUR hen night and HER choice to have a baby. On the other hand, I'd know I'd be gutted if my CBM wasn't on my hen so you'd just have to decide if it's worth compromising on to have her there! And if she can't drink that doesn't really matter at all!
If she falls pregnant so that baby is due AROUND THE TIME of your wedding. I would consider choosing a different CBM and maybe calmly and nicely mention this to her, that if she decided to get pregnant and it happens this way that you'd feel it best that she was just a BM rather than a CBM incase she can't make it/carry out her role fully! If she wants to be your CBM so much maybe she'll have a second thought and delay trying for a bit!
Finally, it may take her longer this time, fertility changes and if she falls pregnant so that the baby is due AFTER your wedding. No problem, okay so you'll have to figure her dress out with a bump possibly and again she may not be able to drink but minor issues!
Whatever the case may be, just remember you love your friend and in any other circumstance you'd be over the moon about her having a baby!
Found Each Other: 23rd April 2005
Engaged: 14th February 2010
Married: 8th September 2012