I posted a quick reply to this yesterday, and I'd like to take a moment to expand on my reply.
Initially we hadn't planned to tell our parents, but the opportunity for us to take a dream vacation came up. In the moment, we were so concerned about the money (we'd be dipping into our baby savings) that we went to DH's parents and told them the deal - that this dream vacation opportunity came up, but that we would have to dip into our baby savings, and we want to start trying really soon, so do they think it's a good idea for us to dip into the savings?
In retrospect, we didn't have to tell them at all, but we were concerned about the money.
It opened up a whole ordeal. My MIL was waiting on my husband's older brother and his wife to start having kids, and as soon as she heard we were going to start, she turned the heat on under them to start. They had announced that they weren't going to have kids, but she started pressuring them to no end to have kids first. She even went so far as to pull me aside and give me a lecture about how I needed to wait, how HER SON isn't ready, all the things I need to change about him before he'll be ready, etc etc etc.
She ended up pushing his older son and his wife so much, that they were forced into a position where they had to go around to family and explain their decision and why, even though they weren't comfortable talking about it, just to get her to back off.
All of that happened because DH and I couldn't keep our mouths shut. I know it would've come up once we got pregnant, but at least then my MIL wouldn't have included the "You're too young to be a mom, but at least you'll be better that one of those 17 year old crack whore moms" conversation. And hopefully their attention would've been on us so much that she wouldn't ave pressured her older son and his wife so much.