A while back I read this post at Clothesline Alley which I thought was very thoughtful and timely. She writes from the point-of-view of someone wanting to have a child/more children, and how that situation feels when others are offering questions or comments on the planning of their family.
I know that some time ago I realized that a lot of what I said to people in this regard might not be received as nicely as it was presented.. That is, while I might be lightly teasing or even happily encouraging (and if I ever DID encourage anyone toward parenthood, be assured it was because I thought well enough of them to do so!), I could not know for sure what the situation was for the other person. Saying, "you should get one of these!" about my baby might not mean the same thing to the recipient of the comment.
For instance, I usually meant, "babies are wonderful, they're blessings, and I hope you will be so blessed." If the couple was actively preventing that blessing, would they take that as encouragement... or judgment? I never imagined it could be considered judgmental, but what if it was? Worse, what if they'd stopped 'preventing' (and I didn't know that) and were dealing with a recent disappointment? Responding with, "yeah, we'd like to..." then establishes an expectation, that the couple is "trying" -- and that's not anyone's business. What if they'd been trying and trying, and each month brought heartache? Wow, my cute little "encouraging" comment just became an arrow in their hearts...
It's best not to assume, and not to comment. Unless someone shares their fertility situation, it's just best 'not to go there.' We cannot know if the couple is making choices, or dealing with unwanted circumstances, and as Amy's post demonstrates, it's best to err on the side of caution.
Another good read on the subject is from Laine's Letters; here, another thought here, and a follow up, here.