I can truly understand your fears about what your parents might think. My MIL was not happy when we told her about our second pregnancy. I don't know what she thought about our first but given her reaction to the second-- I'm sure she wasn't happy with the first either. My MIL actually asked if our second was planned. I was not pleased since she asked right in front of our then 7 yr old.
We have told her that we are trying for a third but we haven't told her the good news yet. We are going to do something cute. I'm going to wrap little gifts with a card and candy inside saying we are expecting. If she becomes upset, then she does. I can't control her emotions, nor can I control how self-centered she has become in past recent years.
I can understand that she wants to make sure we can financially handle children and she's scared that we may be a job loss away from not being able to do that. But I don't want to live in fear. I can't let the "maybes" or "what ifs" run my life and neither should you.
If you feel they will react badly, then I suggest telling as many positive people you know first before you tell them. It might make it a little easier. We did that with our second and we are doing that with our third and it's helping.