Ok, let me start this with saying I have never been pregnant, so I may not know what I'm talking about.
I think this all boils down to your fitness prior to the pregnancy. What were your long runs like before you stopped running? Or, before you started cutting down your long runs? And how long ago did you cut them down/out?
Have you been doing anything to stay fit/strong? Walking, swimming, etc?
And, most importantly, will your spouse/significant other be supportive of you taking the time to train?
I think you're not able to dive right back into training immediately after giving birth. It takes your body awhile to recover from that. Check with your doctor on this, but I'd guess you wouldn't be cleared to run until January. With that said, if you're going into this birth pretty healthy, I think there is no reason at all why you couldn't complete a half marathon by Spring.
As for doing the half ironman - I would give yourself more time. I'm assuming you have at least one of these under your belt already? If not, I'd say no. Remember, the baby is going to be wrecking your sleep, and you're going to be a walking zombie for quite a few months. Now, I've only done one half ironman, so I'm by no means an expert on that either, but by the end of my training, I was averaging 10 hrs of training a week (while working full time), and that made me a walking zombie. I can't imagine doing that while also trying to juggle middle-of-the-night feedings, diaper changings, and all of the other work associated with a newborn. If you've already done a HIM though, I'd say its not unrealistic to think to do a later-season one -- August or September, maybe.
But again, I'm not an expert on any of this -- no kids, just a couple of cats and a husband to take care of, and only one HIM under my belt.
I am curious to hear what you end up doing though, and how it works out!