I guess I'm a little confused, too. If people made firm plans to come and stay with you, and then they simply didn't show up with no official notification (not counting someone who told you that someone else said something to them secretly...) that is incredibly rude and hurtful. You made preparations to feed and house overnight guests, and then they bailed without even letting you know. For all you knew they could have been dead in a ditch somewhere (not to be morbid, that is just something we say in my family).
I too think it would have been best if you had called them on the day they were supposed to arrive--when they would have been late--and asked them what was going on. But, since you can't go back in time, I think you could contact them now to say, "Hey, what happened?" It seems like you aren't really supposed to know about the pregnancy, so I don't think you need to address it--you can address why they stood you up without notice, and say how hurtful this was to you. If they tell you about the pregnancy then, you can say, (cheerful voice) "Wow, that's great, congratulations!" (serious voice) "But I wish you would've at least told me you weren't coming. You didn't have to say why, but I worried about where you were, and I had made all these preparations for you."
And I think you probably have a good point about being pressured to accommodate people who don't go out of their way to accommodate you. At least, this happens to a lot of people I know, and it takes a lot to break those habits and stand up for themselves. Something like standing you up when you've made firm, multi-day holiday plans is a good break point, I think. At least, it worked for my mom--one year she offered to host family Christmas for her in-laws, they all agreed, and then... no one showed up. From among several households. They all just changed their minds--some for legit reasons like weather, some just capriciously--and no one bothered to call her. Some had even gotten to within a few miles of the house before changing their minds. That was the last time she offered to host anything for them.