If your neighbor does say something and you want off the hook, perhaps say something like, "I thought you might be, but I really didn't want to say anything until you did. You know how these things are." Since a lot of women (especially those who've suffered miscarriages or are worried about jobs!) don't announce their pregnancy the instant they get that positive test, this reply is totally understandable, and it gets you off the hook in her thinking you thought she was putting on weight.